Java中的return关键字

在Java中， return是一个保留关键字，即我们不能将它用作标识符。它用于退出一个方法，有或没有一个值。 return 关键字的使用有以下两种方式：

• 案例 1：返回值的方法
• 案例 2：方法不返回值

让我们通过直接实现它们来说明如下：

案例 1：返回值的方法

For methods that define a return type, return statement must be immediately followed by return value. 

例子：

Java

// Java Program to Illustrate Usage of return Keyword
 
// Main method
class GFG {
 
    // Method 1
    // Since return type of RR method is double
    // so this method should return double value
    double RR(double a, double b) {
        double sum = 0;
        sum = (a + b) / 2.0;
       
        // Return statement as we already above have declared
        // return type to be double
        return sum;
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Print statement
        System.out.println(new GFG().RR(5.5, 6.5));
    }
}

输出

6.0

时间复杂度： O(1)

辅助空间： O(1)

输出说明：当 我们正在调用一个具有返回 sum的类 GFG 方法，该方法返回 sum 的值，并且该值将显示在控制台上。

案例 2：方法不返回值

对于不返回值的方法，可以跳过Java中的 return 语句。这里有两种情况，当用户没有返回值时，如下所示：

• #1：在 void函数中不使用 return 语句的方法
• #2：返回类型为 void 的方法 

#1：在 void函数中不使用 return 语句的方法

例子

Java

// Java program to illustrate no return
// keyword needed inside void method
 
// Main class
class GFG {
 
    // Since return type of RR method is
    // void so this method shouldn't return any value
    void demoSum(int a, int b)
    {
        int sum = 0;
        sum = (a + b) / 10;
        System.out.println(sum);
 
        // No return statement in this method
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Calling the method
        // Over custom inputs
        new GFG().demoSum(5, 5);
 
        // Display message on the console for successful
        // execution of the program
        System.out.print(
            "No return keyword is used and program executed successfully");
    }
 
    // Note here we are not returning anything
    // as the return type is void
}

输出

1
No return keyword is used and program executed successfully

Note: Return statement not required (but can be used) for methods with return type void. We can use “return;” which means not return anything. 

#2：返回类型为 void的方法

示例 1-A：

Java

// Java program to illustrate usage of
// return keyword in void method
 
// Class 1
// Main class
class GFG {
 
    // Method 1
    // Since return type of RR method is
    // void so this method should not return any value
    void demofunction(double j)
    {
        if (j < 9)
 
            // return statement below(only using
            // return statement and not returning
            // anything):
            // control exits the method if this
            // condition(i.e, j<9) is true.
            return;
        ++j;
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Calling above method declared in above class
        new GFG().demofunction(5.5);
 
        // Display message on console to illustrate
        // successful execution of program
        System.out.println("Program executed successfully");
    }
}

输出

Program executed successfully

输出说明：如果语句if(j<9)为真，则控制退出该方法并且不执行 RR 方法的其余语句，因此再次返回到main() 方法。

现在继续前进，你一定想知道如果我们在程序末尾使用 return 语句会怎样？

return 语句可以在方法的各个地方使用，但我们需要确保它必须是在方法中执行的最后一条语句。

Note: return statement need not to be last statement in a method, but it must be last statement to execute in a method. 

示例 1-B：

Java

// Java program to illustrate return must not be always
// last statement, but must be last statement
// in a method to execute
 
// Main class
class GFG {
 
    // Method 1
    // Helper method
    // Since return type of RR method is void
    // so this method should not return any value
    void demofunction(double i)
    {
        // Demo condition check
        if (i < 9)
 
            // See here return need not be last
            // statement but must be last statement
            // in a method to execute
            return;
 
        else
            ++i;
    }
 
    // Method 2
    // main driver method
    public static void main(String[] args)
    {
        // Calling the method
        new GFG().demofunction(7);
 
        // Display message to illustrate
        // successful execution of program
        System.out.println("Program executed successfully");
    }
}

输出

Program executed successfully

输出说明：

当条件(i<9)为真时，它执行return语句，因此流程从“demofunction”方法中出来并再次返回到 main。在此之后， return 语句必须是方法中执行的最后一条语句，这意味着在 return 之后定义任何代码是没有意义的，如下所述：

示例 2A

Java

// Java program to illustrate usage of
// statement after return statement
 
// Main class
class GFG {
 
    // Since return type of RR method is void
    // so this method should return any value
    // Method 1
    void demofunction(double j)
    {
        return;
 
        // Here get compile error since can't
        // write any statement after return keyword
 
        ++j;
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
 
        // Calling the above defined function
        new GFG().demofunction(5);
    }
}

输出：

示例 2-B

Java

// Java program to illustrate usage
// of return keyword
 
// Main class
class GFG {
 
    // Since return type of RR method is
    // void so this method should not return any value
    // Method 1
    void demofunction(double val)
    {
 
        // Condition check
        if (val < 0) {
 
            System.out.println(val);
            return;
 
            // System.out.println("oshea");
        }
        else
            ++val;
    }
 
    // Method 2
    // Main drive method
    public static void main(String[] args)
    {
 
        // CAlling the above method
        new GFG().demofunction(-1);
 
        // Display message to illustrate
        // successful execution of program
        System.out.println("Program Executed Successfully");
    }
}

输出

-1.0
Program Executed Successfully

Note: In the above program we do uncomment statements it will throw an error.