zero 、 Preface

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One 、 Title Description

subject ： The complexity of life , Finding as like as two peas is impossible. . But in JAVA in , You can judge whether an object is the same by comparing its member variables . This topic will create 3 A pet cat , By comparing their names 、 Age 、 Weight and color attributes to see if they are the same .

Two 、 Their thinking

Write a class , be known as Cat.

stay Cat Definition in class 4 Member variables , They are names 、 Age 、 Weight and color properties

Provide construction methods to set these property values

rewrite equals() Methods and toString() Method .

• rewrite equals Method is to compare whether two objects are the same
• rewrite toString The method is to directly Output object

Java All classes are Object A direct or indirect subclass of a class .Object Class equals Method , Used to compare whether two objects are the same . The default implementation of this method is to compare whether two objects are the same object .

So when we define a class , It is recommended to rewrite this equals Method .

3、 ... and 、 Code details

public class Cat {
    
    private String name; //  Indicates the name of the cat 
    private int age; //  Indicates the age of the cat 
    private double weight; //  Indicates the weight of the cat 
    private Color color; //  Represents the color of the cat 

    public Cat(String name, int age, double weight, Color color) {
    //  Initialize cat properties 
        this.name = name;
        this.age = age;
        this.weight = weight;
        this.color = color;
    }

    @Override
    public boolean equals(Object obj) {
    //  Use attributes to determine whether cats are the same 
        if (this == obj) {
    //  If two cats are the same object, they are the same 
            return true;
        }
        if (obj == null) {
    //  If one of the two cats is null Is different 
            return false;
        }
        if (getClass() != obj.getClass()) {
    //  If two cats have different types, they are different 
            return false;
        }
        Cat cat = (Cat) obj;
        return name.equals(cat.name) && (age == cat.age)
                && (weight == cat.weight) && (color.equals(cat.color));//  Compare cat attributes 
    }

    @Override
    public String toString() {
    //  rewrite toString() Method 
        StringBuilder sb = new StringBuilder();
        sb.append(" name ：" + name + "\n");
        sb.append(" Age ：" + age + "\n");
        sb.append(" weight ：" + weight + "\n");
        sb.append(" Color ：" + color + "\n");
        return sb.toString();
    }

}

Test class

public class Test {
    
    public static void main(String[] args) {
    
        Cat cat1 = new Cat("Java", 12, 21, Color.BLACK);//  Create a cat 1 Number 
        Cat cat2 = new Cat("C++", 12, 21, Color.WHITE); //  Create a cat 2 Number 
        Cat cat3 = new Cat("Java", 12, 21, Color.BLACK);//  Create a cat 3 Number 
        System.out.println(" The cat 1 Number ：" + cat1);//  Output cat 1 Number 
        System.out.println(" The cat 2 Number ：" + cat2);//  Output cat 2 Number 
        System.out.println(" The cat 3 Number ：" + cat3);//  Output cat 3 Number 
        System.out.println(" The cat 1 Whether the number is with the cat 2 Same number ：" + cat1.equals(cat2));//  Compare whether it is the same 
        System.out.println(" The cat 1 Whether the number is with the cat 3 Same number ：" + cat1.equals(cat3));//  Compare whether it is the same 
    }

}

solution 2： rewrite hashCode() Method

Their thinking

Java The created object is saved in the heap , To speed up the search , Hash lookup used . Is to define a key to map the memory address of the object . When you need to find this object , Find the key directly . You don't have to traverse the heap to find this object .

Write a class , be known as Cat2.

stay Cat2 Definition in class 4 Member variables , They are names 、 Age 、 Weight and color properties

Provide construction methods to set these property values

rewrite equals() Methods and hashCode() Method .

• rewrite equals Method is to compare whether two objects are the same
• rewrite hashCode You can save the same objects in the same location .

A simple method of calculating hash code ：

• rewrite equals Method
• Multiply these member variables by different prime numbers and sum them
• This can be used as a new hash code .

Code details

public class Cat2 {
    
    private String name; //  Indicates the name of the cat 
    private int age; //  Indicates the age of the cat 
    private double weight; //  Indicates the weight of the cat 
    private Color color; //  Represents the color of the cat 

    public Cat2(String name, int age, double weight, Color color) {
    //  Initialize cat properties 
        this.name = name;
        this.age = age;
        this.weight = weight;
        this.color = color;
    }

    @Override
    public boolean equals(Object obj) {
    //  Use attributes to determine whether cats are the same 
        if (this == obj) {
    //  If two cats are the same object, they are the same 
            return true;
        }
        if (obj == null) {
    //  If one of the two cats is null Is different 
            return false;
        }
        if (getClass() != obj.getClass()) {
    //  If two cats have different types, they are different 
            return false;
        }
        Cat2 cat = (Cat2) obj;
        return name.equals(cat.name) && (age == cat.age)
                && (weight == cat.weight) && (color.equals(cat.color));//  Compare cat attributes 
    }

    @Override
    public int hashCode() {
    //  rewrite hashCode() Method 
        return 7 * name.hashCode() + 11 * new Integer(age).hashCode() + 13
                * new Double(weight).hashCode() + 17 * color.hashCode();
    }

}

Test class ：

public class Test2 {
    
        public static void main(String[] args) {
    
            Cat2 cat1 = new Cat2("Java", 12, 21, Color.BLACK); //  Create a cat 1 Number 
            Cat2 cat2 = new Cat2("C++", 12, 21, Color.WHITE); //  Create a cat 2 Number 
            Cat2 cat3 = new Cat2("Java", 12, 21, Color.BLACK); //  Create a cat 3 Number 
            System.out.println(" The cat 1 Hash number of ：" + cat1.hashCode());//  Output cat 1 Hash number of 
            System.out.println(" The cat 2 Hash number of ：" + cat2.hashCode());//  Output cat 2 Hash number of 
            System.out.println(" The cat 3 Hash number of ：" + cat3.hashCode());//  Output cat 3 Hash number of 
            System.out.println(" The cat 1 Whether the number is with the cat 2 Same number ：" + cat1.equals(cat2));//  Compare whether it is the same 
            System.out.println(" The cat 1 Whether the number is with the cat 3 Same number ：" + cat1.equals(cat3));//  Compare whether it is the same 
        }
}

Four 、 Recommendation column

《JAVA From zero to one 》

《JAVA From zero to one 》 Lesson four ： Class and object Foundation

《JAVA From zero to one 》 Lecture 7 ： Advanced object-oriented features

5、 ... and 、 Sample source code download

Pay attention to the official account below. , Restoration and foundation construction + Title Number

Building foundation 31