为自定义类实现hashcode和equals-Implementinghashcodeandequalsforcustomclasses

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Equals and hashCode contract in Java:

We must override hashCode() when we override equals() method, equals method in Java must follow its contract with hashCode method in Java as stated below.

当我们重写equals（）方法时，我们必须覆盖hashCode（），Java中的equals方法必须遵循其与Java中的hashCode方法的契约，如下所述。

1. If two objects are equal by equals() method then there hashcode must be same.
2. 如果两个对象等于equals（）方法，则哈希码必须相同。
3. If two objects are not equal by equals() method then there hashcode could be same or different.
4. 如果两个对象不等于equals（）方法，则哈希码可以相同或不同。

These are sample implementation of equals and hashcode methods for your class:

这些是您的类的equals和hashcode方法的示例实现：

 //Hashcode Implementation    

   @Override
    public int hashCode() 
    {
        final int prime = 31;
        int result = 1;
        result = prime * result + (isActive ? 1231 : 1237);
        result = prime * result + (wasActive ? 1231 : 1237);
        return result;
    }

//equals Implementation    
    @Override
    public boolean equals(Object obj) 
    {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Cell other = (Cell) obj;
        if (isActive != other.isActive)
            return false;
        if (wasActive != other.wasActive)
            return false;
        return true;
    }

1  

You are SOL. Where hashCode() is used in a key in standard Java collections, it should not change. Or else you need a custom HashSet-like implementation.

你是SOL。在标准Java集合中的键中使用hashCode（）的地方，它不应该更改。否则你需要一个类似HashSet的自定义实现。

Use only non-changing fields (or, if you are daring and don't mind occasional crashes, very rarely changing fields) to calculate hashCode().

仅使用不变的字段（或者，如果您大胆并且不介意偶尔崩溃，很少更改字段）来计算hashCode（）。

(Added). In your particular example, use Object.hashCode().

（添加）。在您的特定示例中，使用Object.hashCode（）。

(Added #2) Even if your Cell class were immutable (the two booleans didn't change) it makes a poor choice for hashing, because it only has 2 bits of range. Imagine hashing all people by whether they are male/female and blue eyes / brown eyes. A very good start, but there's only 4 categories, and there will be like 2 billion people in each one. Ideally, you'd have several other categories, like year of birth, country of birth, etc.

（添加＃2）即使你的Cell类是不可变的（两个布尔值没有改变），它也很难选择散列，因为它只有2位的范围。想象一下，所有人都是男性/女性和蓝眼睛/棕色眼睛。一个非常好的开始，但只有4个类别，每个类别将有20亿人。理想情况下，您还有其他几个类别，如出生年份，出生国家等。

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Here is the example of class which has private fields.

以下是具有私有字段的类的示例。

public class Test {

    private int num;
    private String data;

    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if ((obj == null) || (obj.getClass() != this.getClass()))
            return false;
        // object must be Test at this point
        Test test = (Test) obj;
        return num == test.num
                && (data == test.data || (data != null && data
                        .equals(test.data)));
    }

    public int hashCode() {
        int hash = 7;
        hash = 31 * hash + num;
        hash = 31 * hash + (null == data ? 0 : data.hashCode());
        return hash;
    }

}