覆盖java中的hashcode和equals方法？

问题描述

我有以下课程：

public class Sample implements java.io.Serializable{

  //POJO with two fields and getters/setters

   private String name;
   private Integer id;

   //This POJO does not override equals() and hashCode()
}

public class B{
 private Sample sample;

  //here i need override hashcode and equals() based on **sample** property.


}

当我尝试重写equals（）时 B 类中的hashCode（）我在Eclipse中得到了以下错误。

When i tried overriding equals() and hashCode() in the B class I got the error below in Eclipse.

现在如何比较两个 B 实例是否等于或不基于Sample属性？
我无法修改Sample类。

Now how can I compare two B instances whether equals or not based on the Sample property?I cannot modify Sample class.

推荐答案

你看起来像下面这样吗？试试吧，从你的问题开始，我想你也想比较你的Sample类的内容。

are you looking something like following? Just try it, as from your question i think you want to compare contents of your Sample class also.

class Sample implements java.io.Serializable{

    //POJO with two fields and getters/setters

    private String name;
    private Integer id;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }

    //This POJO does not override equals() and hashCode()
}

public class Beta implements Comparable{
    private Sample sample;

    public Sample getSample() {
        return sample;
    }

    public void setSample(Sample sample) {
        this.sample = sample;
    }

    @Override
    public int compareTo(Object o) {

        if(!(o instanceof Beta)){
            return -1;
        }
        }if(((Beta)o).getSample().getName().equals(this.sample.getName())){
                return 0; // return true if names are equal
            }
            if(((Beta)o).getSample().getId().equals(this.sample.getId())){
            //if name are notequal and IDs are equal, do what you need to do
            }
        return -1;
    }

    public static void main(String[] args) {
        Beta b = new Beta();
        Sample s = new Sample();
        s.setId(10);
        s.setName("Name1");
        b.setSample(s);

        Beta b2 = new Beta();
        Sample s2 = new Sample();
        s2.setId(20);
        s2.setName("Name2");
        b2.setSample(s2);

        System.out.println(b2.compareTo(b));

        Beta b3 = new Beta();
        Sample s3 = new Sample();
        s3.setId(10);
        s3.setName("Name1");
        b3.setSample(s3);

        System.out.println(b3.compareTo(b));
    }
}

覆盖方法

class Sample implements java.io.Serializable{

    //POJO with two fields and getters/setters

    private String name;
    private Integer id;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }

    //This POJO does not override equals() and hashCode()
}

public class Beta /*implements Comparable*/{
    private Sample sample;

    public Sample getSample() {
        return sample;
    }

    public void setSample(Sample sample) {
        this.sample = sample;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Beta other = (Beta) obj;

        if ((this.getSample() == null) && (other.getSample() == null)){
            return true;
        }
        if ((this.getSample().getId().equals(other.getSample().getId())) && (this.getSample().getName().equals(other.getSample().getName()))) {
            return true;
        }
        return false;
    }

    @Override
    public int hashCode() {
        int hash = 3;
        hash = 53 * hash + (this.getSample().getName() != null ? this.getSample().getName().hashCode() : 0);
        hash = 53 * hash + (this.getSample().getId() != null ? this.getSample().getId().hashCode() : 0);
        return hash;
    }


/*  @Override
    public int compareTo(Object o) {

        if(!(o instanceof Beta)){
            return -1;
        }
        if(((Beta)o).getSample().getId().equals(this.sample.getId()) && ((Beta)o).getSample().getName().equals(this.sample.getName())){
            return 0;
        }
        return -1;
    }*/

    public static void main(String[] args) {
        Beta b = new Beta();
        Sample s = new Sample();
        s.setId(10);
        s.setName("Name1");
        b.setSample(s);

        Beta b2 = new Beta();
        Sample s2 = new Sample();
        s2.setId(20);
        s2.setName("Name2");
        b2.setSample(s2);

        System.out.println(b2.equals(b));

        Beta b3 = new Beta();
        Sample s3 = new Sample();
        s3.setId(10);
        s3.setName("Name1");
        b3.setSample(s3);

        System.out.println(b3.equals(b));
    }

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