对象引用 方法传参 值传递 引用传递 易错点 MD

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对象引用 方法传参 值传递 引用传递 易错点

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目录

目录
概念
方法传递
案例一
案例二
对象引用
案例一
案例二
案例三
案例四

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); //1，2
        swap(a, b);
        System.out.println(a + "，" + b); //1，2
    }

    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【乱】

        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; //【乱】
        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");
        person1.name = "9";
        person2.name = "10";
    }
}

class Person {
    public String name;

    public Person(String name) {
        this.name = name;
    }
}

对象引用

案例一

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); //只是添加元素
        list3.clear();
        this.list3.addAll(list); //同样只是添加元素
    }
}

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;
        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 work = list.get(0).work;
        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