Java值传递与引用传递
Java值传递与引用传递
Java面试题:
当一个对象被当作参数传递到一个方法后,此方法可改变这个对象的属性,并可返回变化后的结果,那么这里到底是值传递还是引用传递?
答案基本上是:值传递
说明:得出这种结论的前提必须是“参数的值就是对该对象的引用,而不是对象的内容”
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值传递:方法调用时,实际参数把它的值传递给对应的形式参数,方法中执行形式参数值的改变不影响实际参数的值。
引用传递:也称为传地址。方法调用时,实际参数的引用(即地址,而不是参数的值)被传递给方法中相对应的形式参数,
在方法中执行对形式参数的操作实际上就是对实际参数的操作(地址操作),这样方法执行中形式参数值的改变将会影响实际参数的值。
值传递——基本数据类型作为参数(如:String,int)
- public static void changeInt(int a, int b){
- int tmp;
- tmp = a;
- a = b;
- b = tmp;
- }
- // int类型,值传递
- int a = 123;
- int b = 456;
- printInt(a, b);
- changeInt(a, b);
- printInt(a, b);
a = 123; b = 456
a = 123; b = 456
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引用传递——对象或数组作为参数(如:Person,Array)
- class Person{
- String name;
- int age;
- public Person(String name, int age){
- this.name = name;
- this.age = age;
- }
- public void changePerson(Person person, String name, int age){
- person.name = name;
- person.age = age;
- }
- public void printPerson(Person person){
- System.out.println("name = " + person.name + "; age = " + person.age);
- }
- }
- // 对象,引用传递
- Person p = new Person("张三", 20);
- p.printPerson(p);
- p.changePerson(p, "李四", 30);
- p.printPerson(p);
name = 张三; age = 20
name = 李四; age = 30
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完整示例:
- package com.val_ref;
- // Person类
- class Person{
- String name;
- int age;
- public Person(String name, int age){
- this.name = name;
- this.age = age;
- }
- public void changePerson(Person person, String name, int age){
- person.name = name;
- person.age = age;
- }
- public void printPerson(Person person){
- System.out.println("name = " + person.name + "; age = " + person.age);
- }
- }
- class iArray{
- public iArray(){
- }
- public void reverseArray(int []array){
- int tmp;
- int len = array.length;
- for(int i=0; i<len/2; i++){
- tmp = array[i];
- array[i]= array[len-1-i];
- array[len-1-i] = tmp;
- }
- }
- public void printArray(int []array){
- for(int i=0; i<array.length; i++){
- System.out.print(array[i] + " ");
- }
- System.out.println();
- }
- }
- public class testValue {
- public static void main(String []args){
- // 对象,引用传递
- Person p = new Person("张三", 20);
- p.printPerson(p);
- p.changePerson(p, "李四", 30);
- p.printPerson(p);
- // 对象,引用传递
- int []array = new int[5];
- for(int i=0; i<array.length; i++){
- array[i] = i+1;
- }
- iArray obj = new iArray();
- obj.printArray(array);
- obj.reverseArray(array);
- obj.printArray(array);
- // String类型,值传递
- String name1 = "张三";
- String name2 = "李四";
- printString(name1, name2);
- changeString(name1, name2);
- printString(name1, name2);
- // int类型,值传递
- int a = 123;
- int b = 456;
- printInt(a, b);
- changeInt(a, b);
- printInt(a, b);
- }
- // String类型
- public static void changeString(String name1, String name2){
- String tmpName;
- tmpName = name1;
- name1 = name2;
- name2 = tmpName;
- }
- public static void printString(String name1, String name2){
- System.out.println("name1 = " + name1 + "; name2 = " + name2);
- }
- // int类型
- public static void changeInt(int a, int b){
- int tmp;
- tmp = a;
- a = b;
- b = tmp;
- }
- public static void printInt(int a, int b){
- System.out.println("a = " + a + "; b = " + b);
- }
- }
name = 张三; age = 20
name = 李四; age = 30
1 2 3 4 5
5 4 3 2 1
name1 = 张三; name2 = 李四
name1 = 张三; name2 = 李四
a = 123; b = 456
a = 123; b = 456
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