邏輯運算子

Java 中級

Jim White

Java Developer

AND、OR 與 NOT

邏輯運算子可在一個敘述中結合多個條件
三種常見運算子：
- AND
- OR
- NOT

AND

檢查兩個條件是否都為 true

boolean isLoggedIn = true;
boolean isAdmin = true;

// Both are true
if (isLoggedIn && isAdmin) {
  System.out.println("Welcome admin!");
} else {
  System.out.println("You shall not pass.");
}

Welcome admin!

只要有一個為 false，&& 會回傳 false

boolean isLoggedIn = false;
boolean isAdmin = true;

// Only one is true, so else is executed
if (isLoggedIn && isAdmin) {
  System.out.println("Welcome admin!");
} else {
  System.out.println("You shall not pass.");
}

You shall not pass.

OR

檢查是否任一條件為 true

boolean isAdmin = true;
boolean isModerator = false;

// Only one condition is true
if (isAdmin || isModerator) {
  System.out.println("Welcome!");
} else {
  System.out.println("You shall not pass.");
}

Welcome!

只要其中一個或兩個條件為 true 就回傳 true

boolean isAdmin = true;
boolean isModerator = true;

// Both conditions are true
if (isAdmin || isModerator) {
  System.out.println("Welcome!");
} else {
  System.out.println("You shall not pass.");
}

Welcome!

NOT

將 true 變為 false，false 變為 true

boolean isLoggedIn = false;

// isLoggedIn is false, so !isLoggedIn is true
if (!isLoggedIn) {
    System.out.println("You need to log in!");
} else {
    System.out.println("You are logged in, continue!");
}

You need to log in!

邏輯運算子搭配 if-else if-else

int score = 90;
boolean isAttending = false;

if (score > 80 && isAttending) {
  System.out.println("Excellent work!");
} else if (score > 80 && !isAttending) {
  System.out.println("Good work, but attendance is important.");
} else if (score <= 80 && isAttending) {
  System.out.println("You need to work harder.");
} else {
  System.out.println("You need to work harder and attend classes.");
}

Good work, but attendance is important.

重點回顧

下表總結了運算子 &&、|| 與 ! 的值：

`a`	`b`	`a && b`	a \| \| b	`!a`
`true`	`true`	`true`	`true`	`false`
`true`	`false`	`false`	`true`	`false`
`false`	`true`	`false`	`true`	`true`
`false`	`false`	`false`	`false`	`true`

邏輯運算子可用於布林值，或可求值為布林的運算式

boolean a = true && (!false || (true && false)); // a is true
boolean b = (7 <= 3) || (3 >= 5); // b is false

一起來練習吧！

Java 中級