Assignment and Input

The assignment statement

• An assignment statement 赋值语句 puts a value into a variable: x = 5;
• The single = means "store into," not "is equal to."
• Java evaluates the right side first, then stores the result in the left-side variable.
• count = count + 1; reads the old count, adds 1, and stores it back.

Right-to-left flow

• Think of = as a left-pointing arrow: total ← price * qty.
• The variable on the left must already be declared (with a type).
• A later assignment overwrites the old value — the box holds only the newest.
• You can declare and assign in one line: int x = 5;

Assignment copies a value into a variable's box; later changes to one variable do not affect the other

Reading input with Scanner

• To read user input, Java uses a Scanner object tied to System.in.
• Scanner sc = new Scanner(System.in); creates it.
• sc.nextInt() reads an int; sc.nextDouble() a double; sc.next() one word.
• Store what you read in a variable: int age = sc.nextInt();

Match the read to the type

• Use the reading method that matches the type you expect.
• nextInt() for whole numbers, nextDouble() for decimals, nextLine() for a whole line.
• A mismatch (e.g. nextInt() on the text "hello") causes a run-time error.
• Read into a correctly-typed variable so the rest of the code works.