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对象引用 方法传参 值传递 引用传递 易错点
目录
概念
Java中有没有引用传递？
答：Java中只有按值传递，没有按引用传递！
当一个对象被当作参数传递到一个方法中后，在此方法中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：是值传递！
- 不管是原始类型还是引用类型，传递的都是【副本】，也可以说是【值】。
- 如果参数类型是【原始类型】，那么传过来的就是这个参数的值，如果在函数中改变了副本的值不会改变原始的值。
- 如果参数类型是【引用类型】，那么传过来的就是这个引用参数的副本(对象的引用)，这个副本存放的是参数的【地址】。如果在函数中没有改变这个副本的地址，而是改变了地址中的值，那么在函数内的改变会影响到传入的参数；如果在函数中改变了副本的地址，如new一个，那么副本就指向了一个新的地址，此时传入的参数还是指向原来的地址，所以不会改变参数的值。
方法传递
案例一
public class Test {
public static void main(String[] args) {
int a = 1, b = 2;
System.out.println(a + "，" + b);
swap(a, b);
System.out.println(a + "，" + b);
}
public static void swap(int a, int b) {
int tem = a;
a = b;
b = tem;
}
}案例二
public class Test {
public static void main(String[] args) {
Person p1 = new Person("1");
Person p2 = new Person("2");
System.out.println(p1 + " " + p2 + "，" + p1.name + " " + p2.name);//Person@15db9742 Person@6d06d69c，1 2
test(p1, p2);
System.out.println(p1 + " " + p2 + "，" + p1.name + " " + p2.name);//Person@15db9742 Person@6d06d69c，2 1
test2(p1, p2);
System.out.println(p1 + " " + p2 + "，" + p1.name + " " + p2.name);//Person@15db9742 Person@6d06d69c，3 4
test3(p1, p2);
System.out.println(p1 + " " + p2 + "，" + p1.name + " " + p2.name);//Person@15db9742 Person@6d06d69c，6 5
test4(p1, p2);
System.out.println(p1 + " " + p2 + "，" + p1.name + " " + p2.name);//Person@15db9742 Person@6d06d69c，6 5
}
private static void test(Person person1, Person person2) {
String temp = person1.name;
person1.name = person2.name;
person2.name = temp;
}
private static void test2(Person person1, Person person2) {
person1.name = "3";
person2.name = "4";
Person person = person1;
person1 = person2;
person2 = person;
}
private static void test3(Person person1, Person person2) {
Person person = person1;
person1 = person2;
person2 = person;
person1.name = "5";
person2.name = "6";
}
private static void test4(Person person1, Person person2) {
person1 = new Person("7");
person2 = new Person("8");
}
}
class Person {
public String name;
public Person(String name) {
this.name = name;
}
}对象引用
案例一(核心案例)
public class Test {
public static void main(String[] args) {
Work work = new Work();
work.valuse = 1;
Person person = new Person();
person.work = work;
work.valuse = 2;
System.out.println(person.work.valuse); //2
work = new Work();// new 之后 Person 中引用的 Work 就不再是 work 所引用的 Work 了
work.valuse = 3;
System.out.println(person.work.valuse); //2
person.work = work; //将 Person 中引用的 Work 更改为 work 所引用的 Work
System.out.println(person.work.valuse); //3
}
}
class Work {
int valuse;
}
class Person {
Work work;
}案例二
public class Test {
public static void main(String[] args) {
Work work = new Work();
work.valuse = 1;
ArrayList<Work> list = new ArrayList<Work>();
list.add(work);
Person person = new Person();
person.setList(list);
work.valuse = 2;
System.out.println(person.list.get(0).valuse); //2
person.list.get(0).valuse = 3;
System.out.println(list.get(0).valuse); //3
System.out.println(person.list.get(0).valuse); //3
System.out.println("---------以上并没有更改引用的对象地址--------");
work = new Work();
work.valuse = 4;
System.out.println(list.get(0).valuse); //3
System.out.println(person.list.get(0).valuse); //3
list.set(0, work);
System.out.println(list.get(0).valuse); //4
System.out.println(person.list.get(0).valuse); //4
}
}
class Work {
int valuse;
}
class Person {
List<Work> list;
public void setList(List<Work> list) {
this.list = list;
}
}案例三
public class Test {
public static void main(String[] args) {
List<Integer> list = new ArrayList<Integer>();
list.add(1);
A a = new A();
a.setList(list);
list.add(2);
System.out.println(a.list1.toString()); //[1, 2]
System.out.println(a.list2.toString()); //[1]
System.out.println(a.list3.toString()); //[1]
a.setList(list);
System.out.println(a.list1.toString()); //[1, 2]
System.out.println(a.list2.toString()); //[1, 2]
System.out.println(a.list3.toString()); //[1, 2]
}
}
class A {
public List<Integer> list1 = new ArrayList<Integer>();
public List<Integer> list2 = new ArrayList<Integer>();
public List<Integer> list3 = new ArrayList<Integer>();
public void setList(List<Integer> list) {
this.list1 = list;
this.list2 = new ArrayList<Integer>(list);
list3.clear();
this.list3.addAll(list);
}
}案例四
public class Test {
public static void main(String[] args) {
List<Person> list = new ArrayList<>();//一个集合
list.add(new Person("包青天", 1, new Work("百度", 1)));//添加数据
String name = list.get(0).name;
int age = list.get(0).age;
Work work = list.get(0).work;
name = "2";//修改数据。注意，这里的name局部变量和person对象中的name字段指向的是不同的实例对象
age = 2;
work = new Work("2", 2);
System.out.println(list.get(0).name + "," + list.get(0).age + "," + list.get(0).work.company + "," + list.get(0).work.time);//包青天,1,百度,1
list.get(0).name = "3";
list.get(0).age = 3;
list.get(0).work.company = "3";
list.get(0).work.time = 3;
System.out.println(list.get(0).name + "," + list.get(0).age + "," + list.get(0).work.company + "," + list.get(0).work.time);//3,3,3,3
Person temPerson = list.get(0);
temPerson.name = "4";
temPerson.age = 4;
Work tempWork = temPerson.work;
tempWork.company = "4";
tempWork.time = 4;
System.out.println(list.get(0).name + "," + list.get(0).age + "," + list.get(0).work.company + "," + list.get(0).work.time);//4,4,4,4
}
}
class Work {
int time;
String company;
public Work(String company, int time) {
this.time = time;
this.company = company;
}
}
class Person {
int age;
String name;
Work work;
public Person(String name, int age, Work work) {
this.age = age;
this.name = name;
this.work = work;
}
}2017-7-4