Java基础(九)：Object 类的使用

public class Person {
	...
}
//等价于：
public class Person extends Object {
	...
}

int a = 5; 
if(a == 6){…}

Person p1 = new Person();  	    
Person p2 = new Person();
if (p1==p2){…}

public class User {
    private Integer id;
    private String username;
    private String password;

    @Override
    public boolean equals(Object o) {
        // 如果是同一个对象，直接返回true
        if (this == o) {
            return true;
        }
        // 如果传入的对象是null或者不是同一个类，直接返回false
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        // 如果传入的对象是同一个类，所有属性的做比较
        User user = (User) o;
        return Objects.equals(id, user.id)
                && Objects.equals(username, user.username)
                && Objects.equals(password, user.password);
    }
}

int it = 65;
float fl = 65.0f;
System.out.println(“65和65.0f是否相等？” + (it == fl)); // true

char ch1 = 'A'; char ch2 = 12;
System.out.println("65和'A'是否相等？" + (it == ch1)); // true
System.out.println("12和ch2是否相等？" + (12 == ch2)); // true

String str1 = new String("hello");
String str2 = new String("hello");
System.out.println("str1和str2是否相等？"+ (str1 == str2)); // false

System.out.println("str1是否equals str2？"+(str1.equals(str2))); // true

@Data
public class User {
    private Integer id;
    private String username;
    private String password;

    public User(Integer id, String username) {
        this.id = id;
        this.username = username;
    }

    @Override
    public boolean equals(Object o) {
        System.out.println("equals...");
        // 如果是同一个对象，直接返回true
        if (this == o) {
            return true;
        }
        // 如果传入的对象是null或者不是同一个类，直接返回false
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        // 如果传入的对象是同一个类，所有属性的做比较
        User user = (User) o;
        return Objects.equals(id, user.id)
                && Objects.equals(username, user.username)
                && Objects.equals(password, user.password);
    }

    @Override
    public int hashCode() {
    	System.out.println("hashCode...");
        return Objects.hash(id, username, password);
    }
}
// Objects类方法
public static int hash(Object... values) {
    return Arrays.hashCode(values);
}
// Arrays类方法
public static int hashCode(Object a[]) {
    if (a == null){
        return 0;
	}
    int result = 1;

    for (Object element : a){
    	// 31这个数字是为了尽量的减少hash冲突
        result = 31 * result + (element == null ? 0 : element.hashCode());
	}
    return result;
}

public class Test {
    public static void main(String[] args) {
        Set<User> set = new HashSet<>();
        User u1  = new User(3,"张三");
        User u2  = new User(3,"张三");
        set.add(u1);
        set.add(u2);
        System.out.println("set size:"+ set.size());
    }
}

hashCode...
hashCode...
equals...
set size:1

Date now = new Date();
System.out.println(“now=” + now);  //相当于
System.out.println(“now=” + now.toString()); 

s1 = "hello";
System.out.println(s1);//相当于System.out.println(s1.toString());

public class Person {  
    private String name;
    private int age;

    @Override
    public String toString() {
        return "Person{" + "name='" + name + '\'' + ", age=" + age + '}';
    }
}

//Object类的clone()的使用
public class CloneTest {
	public static void main(String[] args) {
		Animal a1 = new Animal("花花");
		try {
			Animal a2 = (Animal) a1.clone();
			// 原始对象：Animal [name=花花]
            System.out.println("原始对象：" + a1); 
            // clone之后的对象：Animal [name=花花]
            System.out.println("clone之后的对象：" + a2); 
		} catch (CloneNotSupportedException e) {
			e.printStackTrace();
		}
	}
}

class Animal implements Cloneable{
	private String name;

	public Animal() {
		super();
	}

	public Animal(String name) {
		super();
		this.name = name;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	@Override
	public String toString() {
		return "Animal [name=" + name + "]";
	}
	
	@Override
	protected Object clone() throws CloneNotSupportedException {
		return super.clone();
	}
	
}

public class FinalizeTest {
	public static void main(String[] args) {
		Person p = new Person("Peter", 12);
		System.out.println(p);
		p = null;//此时对象实体就是垃圾对象，等待被回收。但时间不确定。
		System.gc();//强制性释放空间
	}
}

class Person{
	private String name;
	private int age;

	public Person(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getAge() {
		return age;
	}
	public void setAge(int age) {
		this.age = age;
	}
	//子类重写此方法，可在释放对象前进行某些操作
	@Override
	protected void finalize() throws Throwable {
		System.out.println("对象被释放--->" + this);
	}
	@Override
	public String toString() {
		return "Person [name=" + name + ", age=" + age + "]";
	}
	
}

public static void main(String[] args) {
	Object obj = new Person();
	System.out.println(obj.getClass());//运行时类型
}

class com.atguigu.java.Person