Java学习笔记-按值传递

参数的值传递

实参必须与方法中次你故意的参数在次序和数量上匹配,在类型上兼容.
类型兼容是指不需要经过显式的类型转换,实参的值就可以传递给形参.如将int型的实参值传递给double型形参.

当调用方法时,实参的值传递给形参,这个过程称为按值传递.如果是实参是变量而不是直接量,则将该变量的值传递给形参.无论形参在方法中是否改变,该变量都不受影响.

package welcome;
/*
 * void方法
 */
public class TestVoidMethod {
    public static void main(String[] args) {
        System.out.print("The grade is ");
        printGrade(78.5);   // 对void方法的调用必须是一条语句.
        System.out.print("The grade is ");
        printGrade(59.5);
    }

    public static void printGrade(double score){ // printGrade方法是一个void方法,它不返回任何值
        if(score < 0 || score > 100){
            System.out.println("Invalid score");
            return;   // return用于终止方法,并返回到到方法的调用者
        }

        if(score >= 90.0){
            System.out.println("A");
        }else if(score >= 80.0){
            System.out.println("B");
        }else if(score >= 70.0){
            System.out.println("C");
        }else if(score >= 60.0){
            System.out.println("D");
        }else{
            System.out.println("F");
        }
    }
}

package welcome;
/*
 * 带返回值的方法
 */
public class TestReturnGradeMethod {
    public static void main(String[] args) {
        System.out.println("The grade is " + getGrade(78.5));
        System.out.println("The grade is " + getGrade(59.5));
    }

    public static char getGrade(double score){
        if(score >= 90){
            return 'A';
        }else if(score >= 80){
            return 'B';
        }else if(score >= 70){
            return 'C';
        }else if(score >= 60){
            return 'D';
        }else{
            return 'F';
        }
    }
}

package welcome;
/*
 * Java传参机制是按值传递(pass-by-value)
 * 当调用方法时,实参的值传递给形参,这个过程称为按值传递.
 * 如果是实参是变量而不是直接量,则将该变量的值传递给形参.无论形参在方法中是否改变,该变量都不受影响.
 */
public class Increment {
    public static void main(String[] args) {
        int x = 1;
        System.out.println("Before the call, x is " + x);
        increment(x);
        System.out.println("After the call, x is " + x);
    }

    public static void increment(int n){
        n++;
        System.out.println("n inside the method is " + n);
    }
}

package welcome;

public class TestPassByValue {
    public static void main(String[] args) {
        int num1 = 1;
        int num2 = 2;

        System.out.println("Before invokint the swap method, num1 is " + num1 + " num2 is " + num2);

        swap(num1, num2);

        System.out.println("After invoking the swap method, num1 is " + num1 + " num2 is " + num2);
    }

    public static void swap(int n1, int n2){
        System.out.println("\tInside the swap method");
        System.out.println("\t\tBefore swapping n1 is " + n1 + " n2 is " + n2);

        int temp = n1;
        n1 = n2;
        n2 = temp;

        System.out.println("\t\tAfter swapping n1 is " + n1 + " n2 is " + n2);
    }
}