Java学习笔记——Java语言基础（十一）（Object类）

一、Object类

1.1 API

1.API（Application Programming Interface）应用程序编程接口
2.Java API：Java提供的可以直接使用的类，这些类将底层的实现封装起来。

1.2 Object

1.Object类：类层次结构的根类，所有类都直接或者间接的继承Object类
2.构造方法：public Object（） 子类的构造方法默认访问的是父类的无参构造方法

1.3 Object的hashCode()方法

1.返回对象的哈希吗值，不同对象的hashCode()一般来说不会相同

1.4 Object类的getClass()方法

1.通过getClass()方法，获取对象的真实类的全名称

hashCode()和getClass()的代码如下：

public class Test03 {
    public static void main(String[] args) {
        //Object的hashCode()方法
        Object obj1 = new Object();
        System.out.println(obj1);//[e-mail]
        int o1 = obj1.hashCode();
        System.out.println(o1);//356573597
        Object obj2 = new Object();
        System.out.println(obj2);//[e-mail]
        int o2 = obj2.hashCode();
        System.out.println(o2);//1735600054

        //Object的getClass()方法
        Object obj3 = new Object();
        System.out.println(obj3);//[e-mail]
        Object obj4 = new Object();
        System.out.println(obj4);//[e-mail]
        //比较地址值
        System.out.println(obj3==obj4);//false
        Class C3 = obj3.getClass();
        Class C4 = obj4.getClass();
        //比较类名
        System.out.println(C3);//class java.lang.Object
        System.out.println(C4);//class java.lang.Object
        System.out.println(C3==C4);//true
    }
}

1.5 Object类的toString()方法

1.返回该对象的字符串

public class Test04 {
    public static void main(String[] args) {
        Object obj = new Object();
        String s = obj.toString();
        //打印地址值
        System.out.println(s);//[e-mail]
        /*没有重写toString方法
        Student student1 = new Student();
        String s1 = student1.toString();
        System.out.println(s1);//[e-mail]
        */
        //重写toString方法
        Student student = new Student("张三", 23);
        String s1 = student.toString();
        System.out.println(s1);//Student{name='张三', age=23}
    }
}
class Student{
    String name;
    int age;
    public Student() {
    }
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    //重写toString方法
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

1.6 Object类的equals()方法

1.指示其他某个对象是否与调用方法的对象相等
2.默认情况下比较的是对象的引用是否相同
3.由于比较引用没有意义，一般建议重写equal()方法。

public class Test05 {
    public static void main(String[] args) {
        //Object类的equals方法
        Object obj1 = new Object();
        Object obj2 = new Object();
        System.out.println(obj1.equals(obj2));//false
        System.out.println(obj1==obj2);//fasle
        //Student1重写equals方法
        Student1 s1 = new Student1("张三", 23);
        Student1 s2 = new Student1("张三", 24);
        Student1 s3 = new Student1("张三", 23);
        boolean b1 = s1.equals(s2);
        boolean b2 = s1.equals(s3);
        System.out.println(b1);//false
        System.out.println(b2);//true
    }
}
class Student1{
    String name;
    int age;

    public Student1() {
    }

    public Student1(String name, int age) {
        this.name = name;
        this.age = age;
    }
    //重写eauals方法,比较的是Student里面的成员变量是否相等
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student1 student1 = (Student1) o;
        return age == student1.age &&
                Objects.equals(name, student1.name);
    }
}

1.7 Object类的clone()方法

1.clone()的权限修饰符是受保护的,在用的时候,让该类重写该方法,并把该方法的权限修饰符改为public
2.对象的浅克隆:浅克隆和深克隆，使用clone方法的克隆为浅克隆
3.如果一个对象需要调用clone方法进行克隆，那么该对象所属的类必须实现Cloneable接口。
4.Cloneable接口并没有任何的抽象方法，这种没有任何抽象的接口时一种标记接口

public class Test01 {
    public static void main(String[] args) throws CloneNotSupportedException {
        Student s1 = new Student("学生A", 23);
        System.out.println(s1);//Student{name='学生', age=23}
        //克隆一个Object类
        //CloneNotSupportedException抛出异常
        Object obj = s1.clone();
        //向下转型
        Student s2= (Student) obj;
        s2.age=30;
        s2.name="学生B";
        System.out.println(s2);//Student{name='学生B', age=30}
    }
}
class Student implements Cloneable{
    String name;
    int age;
    //重写toString方法
    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

当一个方法的参数包含引用类型的时候，clone方法的使用如下：

public class Test02 {
    public static void main(String[] args) throws CloneNotSupportedException {
        DogFood dogfood = new DogFood("骨头");
        Dog dog1 = new Dog("一号狗", 3, dogfood);
        dog1.dogFood.name="火腿";
        System.out.println(dog1.dogFood.name);//火腿
        //克隆Dog的dog1对象
        Dog dog2 = (Dog) dog1.clone();
        dog2.dogFood.name="狗粮";
        System.out.println(dog2.dogFood.name);//狗粮
        System.out.println(dog1.dogFood.name);//狗粮
    }
}
class DogFood{
    String name;
    public DogFood(String name){
        this.name=name;
    }
}
class Dog implements Cloneable {
    String name;
    int age;
    DogFood dogFood;

    public Dog(String name, int age, DogFood dogFood) {
        this.name = name;
        this.age = age;
        this.dogFood = dogFood;
    }

    @Override
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
}

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