Every Java program lives inside a class 类. It starts at a method 方法 named main. System.out.println(...) prints a line; System.out.print(...) prints with no new line.
Java is compiled 编译: the compiler 编译器 checks the whole program, then it runs.
A variable 变量 must declare 声明 its type. Common primitive types 基本类型: int (whole number), double (decimal), boolean (true/false), char (one letter).
ไทย
ตัวแปรต้องประกาศชนิดของข้อมูล自己. ชนิดพื้นฐานทั่วไป: int (จำนวนเต็ม), double (ทศนิยม), boolean (true/false), char (ตัวอักษรเดียว).
public class Main {
public static void main(String[] args) {
int age = 17;
double price = 9.99;
boolean passed = true;
System.out.println(age + " " + price + " " + passed);
}
}
1.3
Comments & style
English
A comment 注释 is // (one line) or /* ... */ (a block). Indent the code inside braces { }. Class names start Capitalised; variables and methods use camelCase 驼峰命名 (lowercase first).
Common mistakes
Every statement ends with a semicolon ;.
main must be exactly public static void main(String[] args).
public class Main {
public static void main(String[] args) {
int a = 7, b = 2;
System.out.println(a / b); // 3 (integer division)
System.out.println(a % b); // 1
double x = 7.0 / 2; // 3.5 (one side is double)
System.out.println(x);
}
}
int/int truncates; involve a double for a real quotient
2.2
การใช้ objects: String, Math, wrappers
English
Some values are objects 对象 with methods. String has .length(), .substring(), .toUpperCase(). Math has Math.max, Math.sqrt, Math.pow. Wrapper classes 包装类 (Integer, Double) wrap a primitive — e.g. Integer.parseInt("42").
Math.random() returns a random 随机double from 0.0 up to (but not including) 1.0. Scale it and cast to get whole numbers — this is the AP idiom:
Integer.MAX_VALUE (2147483647) and Integer.MIN_VALUE are the limits of int; going past them wraps around (an overflow).
An object variable that points at no object holds null; calling a method on it throws a NullPointerException.
public class Main {
public static void main(String[] args) {
String s = "Hello";
System.out.println(s.length()); // 5
System.out.println(s.toUpperCase()); // HELLO
System.out.println(Math.max(3, 9)); // 9
System.out.println(Integer.parseInt("42") + 1); // 43
}
}
Math.random() คืนค่า random double จาก 0.0 ไปจนถึง (แต่ไม่รวม) 1.0. ขยายสเกลและ cast เพื่อได้จำนวนเต็ม — นี่คือ AP idiom:
public class Main {
public static void main(String[] args) {
// a random whole number from 1 to 6 (a dice roll)
int roll = (int) (Math.random() * 6) + 1;
System.out.println(roll >= 1 && roll <= 6); // true
}
}
public class Main {
public static void main(String[] args) {
double pi = 3.99;
System.out.println((int) pi); // 3
int total = 7, n = 2;
System.out.println((double) total / n); // 3.5
}
}
if runs a block when a condition 条件 is true; else if and else add more cases. The condition goes in ( ), the block in { }.
ไทย
if รัน block เมื่อเงื่อนไขเป็นจริง; else if และ else เพิ่มกรณีอื่นๆ. เงื่อนไขอยู่ใน ( ), block อยู่ใน { }.
public class Main {
public static void main(String[] args) {
int score = 72;
if (score >= 80) {
System.out.println("A");
} else if (score >= 60) {
System.out.println("B");
} else {
System.out.println("fail");
}
}
}
if เลือกทางที่จริง; else เลือกทางที่เท็จ
3.2
Logic operators & comparisons
English
Compare with ==, !=, <, >, <=, >= — a comparison 比较 gives a boolean. Combine with && (and), || (or), ! (not) — the logical operators 逻辑运算符. For Strings, use .equals(...), not==.
public class Main {
public static void main(String[] args) {
int age = 16;
boolean member = true;
System.out.println(age >= 18 && member); // false
String a = "hi";
System.out.println(a.equals("hi")); // true
}
}
3.3
switch
English
switch chooses among many fixed values. Each case ends with break; default is the fallback.
Common mistakes
A condition must be a boolean; if (x = 5) does not compile (use ==).
Each switch case needs a break;, or control falls through to the next case.
&& and || are the logical operators; & and | are bitwise.
ไทย
switch เลือกจากหลายค่าที่กำหนดไว้. แต่ละ case ลงท้ายด้วย break; default เป็นค่าสำรอง.
public class Main {
public static void main(String[] args) {
int day = 3;
switch (day) {
case 1: System.out.println("Mon"); break;
case 3: System.out.println("Wed"); break;
default: System.out.println("other");
}
}
}
ข้อผิดพลาดที่พบบ่อย
เงื่อนไขต้องเป็น boolean; if (x = 5) ไม่ compile (ใช้ ==).
แต่ละ switch case ต้องมี break;, ถ้าไม่มี control จะไหลไป case ถัดไป.
A while loop repeats while a condition is true. Change something inside, or it loops forever.
ไทย
A while loop ทำซ้ำเมื่อ条件是 true. เปลี่ยนอะไรสักอย่างภายใน, ถ้าไม่亦然它将永远循环.
public class Main {
public static void main(String[] args) {
int n = 1;
while (n <= 3) {
System.out.println(n);
n++;
}
}
}
while ตรวจสอบเงื่อนไขก่อนการวนลูปแต่ละรอบของ body
4.2
ลูป for
English
A for loop packs the start, the condition, and the step into one line. Best when you know the count.
ไทย
A for loop บรรจุการเริ่มต้น, เงื่อนไข, และขั้นตอนในบรรทัดเดียว. ดีที่สุดเมื่อคุณทราบจำนวนรอบ.
public class Main {
public static void main(String[] args) {
for (int i = 0; i < 5; i++) {
System.out.print(i + " ");
}
System.out.println(); // 0 1 2 3 4
}
}
4.3
Accumulation
English
The accumulator 累加器 pattern: start a variable before the loop, then update it each turn.
public class Main {
public static void main(String[] args) {
int total = 0;
for (int i = 1; i <= 5; i++) {
total += i;
}
System.out.println(total); // 15
}
}
4.4
Nested loops
English
A loop inside a loop is a nested loop 嵌套循环. The inner loop runs fully on each turn of the outer one.
Common mistakes
for (int i = 0; i < n; i++) runs n times (0 to n - 1); using <= runs one extra.
Do not put a semicolon right after for (...) or while (...) — it makes an empty loop.
Declare the counter in the for header so its scope ends with the loop.
ไทย
Loop Within a loop คือ nested loop. Inner loop รันเต็มรูปแบบในแต่ละรอบของ outer loop.
public class Main {
public static void main(String[] args) {
for (int r = 0; r < 3; r++) {
for (int c = 0; c < 3; c++) {
System.out.print("*");
}
System.out.println();
}
}
}
ข้อผิดพลาดที่พบบ่อย
for (int i = 0; i < n; i++) รัน n ครั้ง (0 ถึง n - 1); การใช้ <= จะรันเพิ่มอีกหนึ่งครั้ง.
อย่าใส่เครื่องหมายกึ่งทวิภาคต่อท้าย for (...) หรือ while (...) — มันจะทำให้เกิด empty loop.
ประกาศตัวนับใน header ของ for เพื่อให้ scope สิ้นสุดพร้อม loop.
A String 字符串 is text. Useful methods: .length(), .charAt(i), .substring(a, b), .indexOf(x), .toUpperCase(), .equals(...). Strings are immutable 不可变 — each method returns a new String.
The methods you will use every day:
Method
Meaning
Example → result
.length()
how many characters
"Hi".length() → 2
.charAt(i)
one character
"Hi".charAt(0) → H
.substring(a, b)
part, stops before b
"Python".substring(0, 3) → Pyt
.substring(a)
from a to the end
"Python".substring(3) → hon
.indexOf(x)
first position, -1 if absent
"banana".indexOf("na") → 2
.equals(s)
same text?
"hi".equals("hi") → true
.compareTo(s)
order: negative / 0 / positive
"apple".compareTo("banana") → negative
compareTo puts Strings in dictionary order — the AP exam uses it for sorting questions:
ไทย
A String คือข้อความ. Methods ที่有用的: .length(), .charAt(i), .substring(a, b), .indexOf(x), .toUpperCase(), .equals(...). Strings are immutable — Each method returns a new String.
public class Main {
public static void main(String[] args) {
String s = "Python";
System.out.println(s.length()); // 6
System.out.println(s.charAt(0)); // P
System.out.println(s.substring(0, 3)); // Pyt
System.out.println(s.toUpperCase()); // PYTHON
}
}
Methods ที่คุณจะใช้ทุกวัน:
Method
ความหมาย
ตัวอย่าง → ผลลัพธ์
.length()
จำนวน字符
"Hi".length() → 2
.charAt(i)
ตัวอักษรเดียว
"Hi".charAt(0) → H
.substring(a, b)
ส่วน, หยุดก่อน b
"Python".substring(0, 3) → Pyt
.substring(a)
จาก a ไปจนจบ
"Python".substring(3) → hon
.indexOf(x)
ตำแหน่งแรก, -1 ถ้าไม่มี
"banana".indexOf("na") → 2
.equals(s)
ข้อความเดียวกัน?
"hi".equals("hi") → true
.compareTo(s)
ลำดับ: ลบ / 0 / บวก
"apple".compareTo("banana") → ลบ
compareTo วาง Strings ใน dictionary order — AP exam ใช้它为 sorting questions:
public class Main {
public static void main(String[] args) {
String a = "apple", b = "banana";
System.out.println(a.compareTo(b) < 0); // true (apple comes first)
System.out.println(a.compareTo("apple")); // 0 (equal)
}
}
public class Main {
public static void main(String[] args) {
String word = "banana";
int count = 0;
for (int i = 0; i < word.length(); i++) {
if (word.charAt(i) == 'a') count++;
}
System.out.println(count); // 3
}
}
public class Main {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder();
for (int i = 1; i <= 5; i++) {
sb.append(i).append(" ");
}
System.out.println(sb.toString().trim()); // 1 2 3 4 5
}
}
ข้อผิดพลาดที่พบบ่อย
Strings are immutable: s.toUpperCase() return a new String, ดังนั้นเก็บผลลัพธ์.
อ่าน character ด้วย s.charAt(i); ความยาวคือ s.length() (method, with ()).
การสร้าง String ด้วย += ใน big loop ช้า; ใช้ StringBuilder.
An array 数组 holds a fixed number of values of one type. Index from 0, and get the size with .length.
ไทย
An array 数组 holds a fixed number of values of one type. Index from 0, and get the size with .length.
public class Main {
public static void main(String[] args) {
int[] scores = {88, 71, 95};
System.out.println(scores[0]); // 88
System.out.println(scores.length); // 3
scores[1] = 100;
System.out.println(scores[1]); // 100
}
}
A primitive holds its value; an array variable holds a reference to the array object
6.2
Array algorithms
English
Walk the array with a loop to find a max, a total, a count, or to search 查找. A for-each loop (for (int x : a)) reads each value in turn.
ไทย
Walk the array with a loop to find a max, a total, a count, or to search 查找. A for-each loop (for (int x : a)) reads each value in turn.
public class Main {
public static void main(String[] args) {
int[] a = {3, 9, 2, 7};
int max = a[0], total = 0;
for (int x : a) {
if (x > max) max = x;
total += x;
}
System.out.println(max + " " + total); // 9 21
}
}
6.3
2-D arrays
English
A 2-D array 二维数组 is a grid 网格 of rows and columns: grid[row][col].
Common mistakes
An array's size is a.length (no brackets, no ()), and it is fixed when created.
Valid indexes are 0 to a.length - 1; a[a.length] throws ArrayIndexOutOfBoundsException.
A new int[5] is filled with zeros, not left empty.
ไทย
A 2-D array 二维数组 is a grid 网格 of rows and columns: grid[row][col].
public class Main {
public static void main(String[] args) {
int[][] grid = {{1, 2, 3}, {4, 5, 6}};
System.out.println(grid[1][2]); // 6
for (int[] row : grid) {
for (int v : row) System.out.print(v + " ");
}
System.out.println(); // 1 2 3 4 5 6
}
}
Common mistakes
An array's size is a.length (no brackets, no ()), and it is fixed when created.
Valid indexes are 0 to a.length - 1; a[a.length] throws ArrayIndexOutOfBoundsException.
A new int[5] is filled with zeros, not left empty.
An ArrayList is a resizable 可变大小 list — it grows and shrinks as you add or remove items. It stores objects, so use a wrapper 包装类 type like Integer (not int). The <Integer> part is a generic 泛型 type. Key methods: .add(x), .get(i), .set(i, x), .size(), .remove(i).
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<Integer> nums = new ArrayList<Integer>();
nums.add(10);
nums.add(20);
nums.add(30);
System.out.println(nums.size()); // 3
System.out.println(nums.get(1)); // 20
nums.set(0, 99);
System.out.println(nums); // [99, 20, 30]
}
}
ArrayList: add, get, size on a resizable list
7.2
อัลกอริทึมของ ArrayList & บักการลบ
English
.remove(i) shifts 移动 every later element one place left. If you remove while counting iup, you skip the next element. Fix: loop backwards, or don't increment i after a remove.
Common mistakes
ArrayList uses .size(), .get(i) and .add(...) — not the [] you use on arrays.
Removing items while looping forward by index skips the next item (the remove bug). Loop backwards, or use an iterator.
Store objects, not primitives: use ArrayList<Integer>, and Java auto-boxes int values.
import java.util.ArrayList;
public class Main {
public static void main(String[] args) {
ArrayList<Integer> nums = new ArrayList<Integer>();
for (int n : new int[]{4, 7, 4, 9, 4}) nums.add(n);
// Remove every 4 — loop backwards so removals don't skip items.
for (int i = nums.size() - 1; i >= 0; i--) {
if (nums.get(i) == 4) nums.remove(i);
}
System.out.println(nums); // [7, 9]
}
}
A class 类 is a blueprint for objects. Its fields 字段 store data, its constructor 构造方法 sets up a new object, and its methods 方法 are the actions. this.name means "this object's name". Create an object with new.
A static member belongs to the class itself, not to any one object. Call a static method on the class name — like Math.max — and a static field is shared by every object:
public class Main {
public static void main(String[] args) {
Dog d = new Dog("Rex", 3);
System.out.println(d.describe()); // Rex is 3 years old
d.haveBirthday();
System.out.println(d.describe()); // Rex is 4 years old
}
}
class Dog {
private String name;
private int age;
public Dog(String name, int age) { // constructor
this.name = name;
this.age = age;
}
public String describe() {
return name + " is " + age + " years old";
}
public void haveBirthday() {
age++;
}
}
public class Main {
public static void main(String[] args) {
System.out.println(Counter.made()); // 0
new Counter();
new Counter();
System.out.println(Counter.made()); // 2
}
}
class Counter {
private static int count = 0; // shared by ALL Counter objects
public Counter() { count++; }
public static int made() { // called on the class: Counter.made()
return count;
}
}
Fields store state; constructor builds; methods act
8.2
Encapsulation (การห่อหุ้มข้อมูล)
English
Encapsulation 封装 means hiding data behind methods. Mark fields private so outside code can't touch them directly; expose an accessor 访问方法 (getter) to read, and a method to change them safely. The method can guard the data — here a deposit must be positive.
Common mistakes
A constructor has the class name and no return type (not even void).
Use this.field to tell a field apart from a parameter with the same name.
Make fields private and reach them through getter/setter methods (encapsulation).
public class Main {
public static void main(String[] args) {
Account a = new Account(100);
a.deposit(50);
a.deposit(-999); // rejected by the guard
System.out.println(a.getBalance()); // 150
}
}
class Account {
private int balance; // hidden from outside
public Account(int start) {
balance = start;
}
public void deposit(int amount) {
if (amount > 0) balance += amount; // guard keeps balance valid
}
public int getBalance() { // accessor (getter)
return balance;
}
}
ข้อผิดพลาดที่พบบ่อย
Constructor มีชื่อคลาสและไม่มีการ return type (แม้แต่ void ก็ไม่มี)
Inheritance 继承 lets a subclass 子类 reuse a superclass 父类. Write class Cat extends Animal and Cat gets Animal's fields and methods for free. Call the parent constructor with super(...).
public class Main {
public static void main(String[] args) {
Cat c = new Cat("Milo");
c.eat(); // Milo is eating (inherited from Animal)
c.speak(); // Meow (Cat's own method)
}
}
class Animal {
protected String name;
public Animal(String name) { this.name = name; }
public void eat() { System.out.println(name + " is eating"); }
}
class Cat extends Animal {
public Cat(String name) { super(name); } // call Animal's constructor
public void speak() { System.out.println("Meow"); }
}
Cat และ Dog extend Animal: พวกมันสืบทอดสมาชิกจาก parent และเพิ่มของตัวเอง
9.2
Polymorphism & toString
English
A subclass can override 重写 a method to replace the parent's version. Polymorphism 多态 means a Shape variable can hold any subtype, and Java picks the right toString at run time. System.out.println(obj) automatically calls obj.toString().
Common mistakes
An overriding method must match the signature exactly; add @Override so the compiler catches slips.
super(...) must be the first line of a subclass constructor.
A subclass object IS-A superclass object, but not the other way round.
public class Main {
public static void main(String[] args) {
Shape[] shapes = { new Circle(2), new Square(3) };
for (Shape s : shapes) {
System.out.println(s); // each calls its own toString
}
}
}
class Shape {
public String toString() { return "a shape"; }
}
class Circle extends Shape {
private int r;
public Circle(int r) { this.r = r; }
public String toString() { return "Circle r=" + r; } // override
}
class Square extends Shape {
private int side;
public Square(int side) { this.side = side; }
public String toString() { return "Square side=" + side; } // override
}
Recursion 递归 is a method that calls itself. Every recursion needs a base case 基准情形 (when to stop) and a recursive call 递归调用 that moves toward it. Without a base case it never stops and crashes with a stack overflow.
Trace it: factorial(5) waits for factorial(4), which waits for factorial(3)… down to factorial(1) returning 1. Then the answers multiply back up: 1 → 2 → 6 → 24 → 120.
Recursion also works on Strings — peel off one character each call:
Merge sort 归并排序 is the recursive sort on the AP exam: split the array in half, sort each half recursively, then merge 合并 the two sorted halves. It runs in O(n log n) — far faster than the O(n²) sorts on big arrays.
Common mistakes
Recursion needs a base case, or it throws StackOverflowError.
Each recursive call must move CLOSER to the base case.
Trace a small example by hand to check the recursion returns the right value.
In merge sort, the merge step does the real work; the recursion only splits the array.
ไทย
Recursion คือเมธอดที่เรียกตัวเอง ทุก recursion ต้องการ base case (จุดหยุด) และ recursive call ที่เข้าใกล้มัน หากไม่มี base case จะไม่หยุดและ crash ด้วย stack overflow
public class Main {
public static void main(String[] args) {
System.out.println(factorial(5)); // 120
}
public static int factorial(int n) {
if (n <= 1) return 1; // base case
return n * factorial(n - 1); // recursive call: n * (n-1)!
}
}
public class Main {
public static void main(String[] args) {
System.out.println(reverse("PYTHON")); // NOHTYP
}
static String reverse(String s) {
if (s.length() <= 1) return s; // base case
return reverse(s.substring(1)) + s.charAt(0);
}
}
import java.util.Arrays;
public class Main {
public static void main(String[] args) {
int[] a = {5, 2, 9, 1, 7, 3};
mergeSort(a, 0, a.length - 1);
System.out.println(Arrays.toString(a)); // [1, 2, 3, 5, 7, 9]
}
static void mergeSort(int[] a, int lo, int hi) {
if (lo >= hi) return; // base case: one element
int mid = (lo + hi) / 2;
mergeSort(a, lo, mid); // sort the left half
mergeSort(a, mid + 1, hi); // sort the right half
merge(a, lo, mid, hi); // merge the two halves
}
static void merge(int[] a, int lo, int mid, int hi) {
int[] tmp = new int[hi - lo + 1];
int i = lo, j = mid + 1, k = 0;
while (i <= mid && j <= hi) {
if (a[i] <= a[j]) tmp[k++] = a[i++];
else tmp[k++] = a[j++];
}
while (i <= mid) tmp[k++] = a[i++];
while (j <= hi) tmp[k++] = a[j++];
for (k = 0; k < tmp.length; k++) a[lo + k] = tmp[k];
}
}
Linear search 线性查找 checks every element — works on any array. Binary search 二分查找 is much faster but needs a sorted 已排序 array: it looks at the middle, then throws away half each step. Both return the index, or -1 if not found.
public class Main {
public static void main(String[] args) {
int[] a = {2, 5, 8, 12, 16, 23}; // sorted, so binary search works
System.out.println(linear(a, 12)); // 3
System.out.println(binary(a, 12)); // 3
System.out.println(binary(a, 9)); // -1 (not found)
}
static int linear(int[] a, int target) {
for (int i = 0; i < a.length; i++)
if (a[i] == target) return i;
return -1;
}
static int binary(int[] a, int target) {
int lo = 0, hi = a.length - 1;
while (lo <= hi) {
int mid = (lo + hi) / 2;
if (a[mid] == target) return mid;
else if (a[mid] < target) lo = mid + 1;
else hi = mid - 1;
}
return -1;
}
}
Selection sort 选择排序 repeatedly finds the smallest remaining value and swaps it to the front. Insertion sort 插入排序 takes each value and slides it back into its place among the already-sorted values.
How the speeds compare:
Algorithm
Time
linear search
O(n)
binary search
O(log n), sorted arrays only
selection / insertion sort
O(n²)
merge sort (topic 10)
O(n log n)
Common mistakes
Binary search only works on a sorted array.
Linear search is O(n); binary search is O(log n) but needs the sort first.
Selection and insertion sort are O(n²) — clear to learn, slow on big data.
import java.util.Arrays;
public class Main {
public static void main(String[] args) {
int[] a = {5, 2, 9, 1, 7};
for (int i = 0; i < a.length - 1; i++) {
int min = i;
for (int j = i + 1; j < a.length; j++)
if (a[j] < a[min]) min = j;
int t = a[min]; a[min] = a[i]; a[i] = t; // swap into place
}
System.out.println(Arrays.toString(a)); // [1, 2, 5, 7, 9]
}
}
import java.util.Arrays;
public class Main {
public static void main(String[] args) {
int[] a = {5, 2, 9, 1, 7};
for (int i = 1; i < a.length; i++) {
int key = a[i], j = i - 1;
while (j >= 0 && a[j] > key) { // shift bigger values right
a[j + 1] = a[j];
j--;
}
a[j + 1] = key; // drop key into the gap
}
System.out.println(Arrays.toString(a)); // [1, 2, 5, 7, 9]
}
}
A Scanner reads text one line at a time. For a real file you write new Scanner(new File("scores.txt")); here we wrap a String so the example runs anywhere. Use .split(" ") to split 拆分 a line into parts and Integer.parseInt(...) to parse 解析 a number from text.
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
// Real file: Scanner in = new Scanner(new File("scores.txt"));
String data = "Alice 80\nBob 95\nCara 72";
Scanner in = new Scanner(data);
int total = 0, count = 0;
while (in.hasNextLine()) {
String line = in.nextLine();
String[] parts = line.split(" "); // break the line on the space
total += Integer.parseInt(parts[1]);
count++;
}
System.out.println("average = " + (total / count)); // average = 82
}
}
Scanner reads lines while hasNextLine is true
12.2
ประเภทคำถาม FRQ ของ AP
English
The AP CS A exam has four free-response 自由作答 questions, each a fixed shape:
Q1 — Methods & control: write methods to a given spec; loops, if, String/Math.
Q2 — Class design: write a full class (fields, constructor, methods) from a description.
Q3 — Array / ArrayList: process a 1-D array or ArrayList (search, count, build a new list).
Q4 — 2-D array: traverse a grid by row and column.
The skill is always the same: read the spec, write the method exactly as described, return the right type.
Common mistakes
nextInt() leaves the newline behind, so a following nextLine() reads an empty line — read it away first.
Check hasNext() before reading, to avoid running off the end of the file.
In the FRQ, read the method header carefully: match the return type and parameters exactly.
ไทย
การสอบ AP CS A มีคำถามแบบตอบเอง (free-response) สี่ข้อ แต่ละข้อมีรูปแบบคงที่ดังนี้:
ทักษะที่ใช้คือสิ่งเดิมเสมอ: อ่านสเปก, เขียน method ตามที่ระบุไว้พอดี, Return type ที่ถูกต้อง
public class Main {
public static void main(String[] args) {
// Q1 style: implement a method to a spec, then it is tested.
System.out.println(countEven(new int[]{4, 7, 10, 3, 6})); // 3
}
/** Returns how many values in arr are even. */
public static int countEven(int[] arr) {
int count = 0;
for (int x : arr)
if (x % 2 == 0) count++;
return count;
}
}
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