How to use Java Object class

1.equals method

==Operator

Comparison operator, which can determine both basic types and reference types

If you determine the basic type, determine whether the values ​​are equal

If you determine the reference type, determine whether the addresses are equal, that is, determine whether they are the same object

equals

is a method of the object class, which can only determine the reference type

object - equals source code:

public boolean equals(Object obj) {
    return (this == obj);
}

It can be clearly seen that the equals method in the object class is to determine whether the address of the object is Identical (is it the same object)

However, other data type classes will override the equals method, such as the rewriting of the String class: (determine whether the values ​​​​of two strings are equal)

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    return (anObject instanceof String aString)
            && (!COMPACT_STRINGS || this.coder == aString.coder)
            && StringLatin1.equals(value, aString.value);
}

Example:

String str1 = new String("hello");
String str2 = new String("hello");
System.out.println(str1 == str2); // false
System.out.println(str1.equals(str2)); // true

2.hashCode

Improve the efficiency of containers with hash structures

If two references point to the same object, the hash value is certain (no conflict) case) is the same, and vice versa

The hash value is based on the address but not the address

Demo:

// hashCode
A a = new A();
A a1 = new A();
A a2 = a;
System.out.println(a.hashCode());
System.out.println(a1.hashCode());
System.out.println(a2.hashCode());
------------------------------

Output:

1324119927
990368553
1324119927

3.toString

Returns the string representation of the object

Source code:

public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

Example:

// toString
A a3 = new A("dahe",25,"安全服务工程师");
System.out.println(a3.toString());

Output:

classes.A@41629346
Package name.Class name@Hex hashCode

Now Let’s rewrite the toString method (template) in the class:

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

Run the above code again, and now the output is as follows:

A{name='dahe', age=25, job='安全服务工程师'}

In addition, when we directly output the object, it will be called directly by default toString method:

System.out.println(a3);

Output:

A{name='dahe', age=25, job='Security Service Engineer'}

4.finalize

When the object is recycled, the system automatically calls the finalize method of the object. Subclasses can override this method and do some operations to release resources

Note: In JDK18. finalize is deprecated. Although it can help us proactively release the underlying resources of the system, to be honest, I have never used it. Java automatically manages memory. Using it will lead to potential system security risks. If it is not helpful, it will be a burden, so it is planned to be removed. .

Example:

// finalize
A a4 = new A("dh",33,"架构师");
a4 = null; // 这是a4成为了垃圾，垃圾回收器就会回收对象
// 在销毁对象之前，会调用对象的finalize方法
// 程序员就可以在这个方法中写入自己的业务，释放资源

Override the finalize method:

@Override
protected void finalize() throws Throwable {
    System.out.println("我们销毁对象");
}

In addition, you can also actively run the garbage collector:

System.gc(); // 主动调用垃圾回收器

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