Statement

The articles are all my technical notes. Please indicate the source for reprinting https://segmentfault.com/u/yzwall

Introduction to Object class

Object class is the parent class of all classes. In Java, only basic data types are not objects. All array types (object type && basic data type array) are inherited from the Object class;

equals method

In the Object class, two objects are judged by judging whether they have the same reference. Whether the objects are the same;
As long as the subclass overrides the equals method, it must override the hashCode method

// in java.lang.Object
public boolean equals(Object obj) {
    return (this == obj);
}

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Principles for rewriting the equals method:

Reflexivity: A.equals(A) returns true;
Symmetry: The result of A.equals(B) is sum B.equals(A) is the same;
Transitiveness: A.equals(B) is true, B.equals(C) is true, then A. equals(C) is true
Consistency: For non-null references A and B, as long as the object information used in the equals() comparison operation remains unchanged, multiple times Calling A.equals(B), the results are consistent;
For any non-null reference, x.equals(null) must return false;
When overriding the equals method, the parameter type must be the Object type

Example of overriding the equals method

class myObject {
    private String name;
    private int age;
    ...
    public getName() {
        return this.name;
    }
    public getAge() {
        return this.age;
    }
}

/**
 * 重写equals方法demo步骤
 * Effective Java中推荐方式
 */
public boolean equals(Object x) {
    // 1. 检查x和this是否引用同一个对象
    if (x == this) {
        return true;
    }
    
    // 2. 检查x对象类型是否是myObject派生
    if (!(x instanceof myObject)) {
        return false;
    }

    // 3. 比较数据域
    // 经过1，2检查，将参数转换为正确类型
    myObject o = (myObject)(x);
    return this.name.equals(x.getName()) && this.age == (x.getAge());
}

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hashCode() method

hashCode methodReturns the hash code of the object, Equal objects must return equal hashCode, and the hashCode of different objects are as unequal as possible;

// in java.lang.Object
public native int hashCode();

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Rewrite Always rewrite hashCode when equals

Rewriting equals without rewriting hashCode will result in"unequal objects have the same hashCode", resulting in collection classesHashMap, HashSet and Hashtable will not work; in the extreme case, make the hashCode of all objects equal in the hash table and all objects will be mapped to the same bucket , the hash table degenerates into a linked list;
When two objects call equal and return true, then the two objects each call hashCode() and return the samehashCode;
When two objects call equal and return false, the hashCode returned by the two objects each calling hashCode() can be the same (Hash conflicts cannot be completely avoided )

toString() method

The toString method in the Object class outputs the object's "object class name@hash code";

Java Object class method analysis

Statement

The articles are all my technical notes. Please indicate the source for reprinting https://segmentfault.com/u/ yzwall

Introduction to Object class

The Object class is the parent class of all classes. In Java, only basic data types are not objects. All array types (object types && basic data type arrays) are inherited from the Object class;

equals method

The Object class determines whether two objects have the same reference. Whether the objects are the same;
As long as the subclass overrides the equals method, it must override the hashCode method

// in java.lang.Object
public boolean equals(Object obj) {
    return (this == obj);
}

Copy after login

Principles for rewriting the equals method:

Reflexivity: A.equals(A) returns true;
Symmetry: The result of A.equals(B) is sum B.equals(A) is the same;
Transitiveness: A.equals(B) is true, B.equals(C) is true, then A. equals(C) is true
Consistency: For non-null references A and B, as long as the object information used in the equals() comparison operation remains unchanged, multiple times Calling A.equals(B), the results are consistent;
For any non-null reference, x.equals(null) must return false;
When overriding the equals method, the parameter type must be the Object type

Example of overriding the equals method

class myObject {
    private String name;
    private int age;
    ...
    public getName() {
        return this.name;
    }
    public getAge() {
        return this.age;
    }
}

/**
 * 重写equals方法demo步骤
 * Effective Java中推荐方式
 */
public boolean equals(Object x) {
    // 1. 检查x和this是否引用同一个对象
    if (x == this) {
        return true;
    }
    
    // 2. 检查x对象类型是否是myObject派生
    if (!(x instanceof myObject)) {
        return false;
    }

    // 3. 比较数据域
    // 经过1，2检查，将参数转换为正确类型
    myObject o = (myObject)(x);
    return this.name.equals(x.getName()) && this.age == (x.getAge());
}

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hashCode() method

hashCode methodReturns the hash code of the object, Equal objects must return equal hashCode, and the hashCode of different objects are as unequal as possible;

// in java.lang.Object
public native int hashCode();

Copy after login

Rewrite Always rewrite hashCode when equals

Rewriting equals without rewriting hashCode will result in"unequal objects have the same hashCode", resulting in collection classesHashMap, HashSet and Hashtable will not work; in the extreme case, make the hashCode of all objects equal in the hash table and all objects will be mapped to the same bucket , the hash table degenerates into a linked list;
When two objects call equal and return true, then the two objects each call hashCode() and return the samehashCode;
When two objects call equal and return false, the hashCode returned by the two objects each calling hashCode() can be the same (Hash conflicts cannot be completely avoided )

toString() method

The toString method in the Object class outputs the object's "object class name@hash code";

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