Java方法返回值：void与非void方法，返回数据正确姿势

What is a Method Return Value?¶

Imagine you have a “calculator tool” that helps you add two numbers. When you input two numbers (e.g., 3 and 5), it gives you a result (8) — this “result” is the method’s return value. In Java, a method is like this tool: it can accept some “input” (parameters) and then possibly return an “output” (return value) to the caller.

I. void Methods: Methods That Return No Data¶

If a method doesn’t need to return any data to the outside, use void as the return type. Its key characteristic is: It ends after execution and has no “output”.

1. Definition Format¶

// Method name: sayHello
// Parameters: None
// Return type: void (no data returned)
public static void methodName() {
    // Method body: Code to execute (e.g., print, modify data)
    System.out.println("Hello, this is a void method!");
}

2. Calling Method¶

Since void methods return no value, you simply call the method name without needing to capture the return value:

public static void main(String[] args) {
    sayHello(); // Call the void method directly; no return value to handle
}

3. Use Cases¶

• When only an action needs to be performed (no result needed): e.g., print a message, save data to a file, initialize an object.
• Example:

// Print user info without returning a value
public static void printUserInfo(String name, int age) {
    System.out.println("Name: " + name + ", Age: " + age);
}

// Call it
printUserInfo("Xiaoming", 18); // Output: Name: Xiaoming, Age: 18

II. Non-void Methods: Methods That Return Data¶

If a method needs to return a result (e.g., a calculation result or query result), use a non-void type (e.g., int, String, double). Its key characteristic is: It must return a value consistent with the declared type.

1. Definition Format¶

// Method name: add
// Parameters: Two int numbers a and b
// Return type: int (returns the sum as an integer)
public static int add(int a, int b) {
    int sum = a + b;
    return sum; // return followed by the data to be returned
}

2. Calling Method¶

Non-void methods return data, so you need to store it in a variable or use it directly in operations:

public static void main(String[] args) {
    // 1. Store the return value in a variable
    int result = add(3, 5); 
    System.out.println("Sum of 3+5: " + result); // Output: Sum of 3+5: 8

    // 2. Use the return value directly in calculations
    int total = add(result, 10); 
    System.out.println("Total sum: " + total); // Output: Total sum: 18
}

3. Use Cases¶

• When a calculation result is needed: e.g., sum, difference, average.
• When a query result is needed: e.g., find the maximum value in an array, count elements in a list.
• Example:

// Find the larger of two numbers
public static int max(int a, int b) {
    if (a > b) {
        return a; // Return a if it's larger
    } else {
        return b; // Return b if it's larger
    }
}

// Call it
int maxNum = max(10, 20); 
System.out.println("Larger number: " + maxNum); // Output: Larger number: 20

III. Correct Practices for Returning Data¶

When writing non-void methods, follow these key points to avoid errors or logical issues:

1. Must Use return to Return Data¶

Non-void methods cannot omit the return statement. If a method declares a return type of int but forgets the return, the compiler will error: “missing return statement”.

// Error: No return statement
public static int add(int a, int b) {
    int sum = a + b; 
    // Forgot to write: return sum; This will cause a compile error!
}

2. Return Type Must Match¶

The method’s declared return type (e.g., int, String) must match (or be compatible with) the data returned by return.
- Error Example 1: Return type int but return a String

public static int getString() {
    return "This is a string"; // Error: Incompatible return type (String vs. int)
}

• Error Example 2: Return type double but return an int (e.g., implicit conversion is allowed but not recommended)

public static double calculate() {
    return 10; // While int is implicitly converted to double, explicitly return 10.0 for clarity
}

3. Unified Return Types in Multiple Branches¶

If a method has multiple return statements (e.g., if-else branches), the returned data must be of the same type:

// Error: Mixed return types (String vs. int)
public static Object getValue() {
    if (true) {
        return "String"; 
    } else {
        return 100; // Error: Inconsistent return types
    }
}

4. return; Can Exit a void Method Early¶

Although void methods don’t need to return values, you can use return; to exit the method early (without returning data):

public static void checkLogin(String name) {
    if (name.isEmpty()) {
        System.out.println("Username cannot be empty!");
        return; // Exit method early; subsequent code won't execute
    }
    System.out.println("Login successful!");
}

IV. Summary¶

• Choose void or non-void: Use void if no data needs to be returned (e.g., printing messages). Use non-void if a result is required (e.g., calculations, queries).
• Non-void methods: Must include a return statement, with a return type matching the declared type. Callers should store the result in a variable or use it directly.
• Avoid common errors: Forgetting return, mismatched return types, or inconsistent types in multiple branches.

Master these rules to use Java method return values correctly and write clear, logical code!