How to use question mark in java

The question mark in Java can be used for the following purposes: Ternary operator: used for conditional judgment, returning different values ​​​​according to conditions. Null value judgment and assignment: Used to judge whether a variable is empty, and return different values ​​or assign values ​​according to conditions. Method's varargs arguments: Convert a variable number of arguments to an array. Destructuring in pattern matching: Deconstructing an object or array into multiple variables.

Uses of question mark in Java

The question mark in Java has many uses, including:

1. Conditional operator (ternary operator)

The ternary operator is a concise conditional expression with the following syntax:

(条件) ? 值1 : 值2;

If the condition is true , then return value 1; otherwise return value 2.

Example:

int age = 18;
String result = (age >= 18) ? "成年" : "未成年";

2. Null value judgment and assignment

Null value judgment:

Question marks can be used to determine whether a variable is null. The syntax is as follows:

变量 != null ? 变量 : 默认值;

If the variable is not null, return the variable value; otherwise, return the default value.

Example:

String name = null;
String displayName = name != null ? name : "未知";

Assignment:

Question marks can also be used to assign values ​​to variables based on conditions. The syntax is as follows:

变量 = 条件 ? 值1 : 值2;

If the condition is true, assign the value 1 to the variable; otherwise, assign the value 2 to the variable.

Example:

boolean isAdult = true;
int ageLimit = isAdult ? 18 : 16;

3. The varargs parameter of the method

The varargs parameter of the method uses three dots (.. .) represents a variable number of parameters, the syntax is as follows:

public static void myMethod(String... args) { ... }

Question mark can convert varargs parameters into an array, the syntax is as follows:

String[] argsArray = myMethod(?);

4. Destructuring in pattern matching

Java 17 introduced pattern matching, where question marks are used to destructure objects or arrays. The syntax is as follows:

var (a, b) = new int[] { 1, 2 };

This code deconstructs new int[] { 1, 2 } into two integers type variables a and b.

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