Vòng lặp while

Repeat while true

• A while loop 当型循环 repeats its body as long as a condition stays true.
• while (i <= 5) { sum = sum + i; i++; }
• Before each pass, Java checks the condition; if false, the loop ends.
• Use while when you don't know in advance how many passes you'll need.

Test-before-each-pass

• The condition is tested before the body runs each time.
• If it's false on the very first check, the body runs zero times.
• After each pass, the condition is re-checked before the next.
• This "check, then maybe run" order is worth memorizing.

The three loop types differ in where the condition is tested

The loop control variable

• A loop control variable 循环控制变量 drives progress toward stopping.
• Initialize it before the loop, use it in the condition, and change it inside the body.
• i starts at 1, the condition checks i <= 5, and i++ moves it forward.
• Without a change that eventually breaks the condition, the loop never ends.

Infinite loops

• An infinite loop 无限循环 happens when the condition never becomes false.
• Forgetting i++ (or updating the wrong variable) traps the program forever.
• The tell: the loop makes no progress toward its stopping condition.
• Always make sure each pass moves the control variable toward the exit.