Java:在equals检查中避免NullPointerException的干净方法

Question

我有一个地址对象,我想为其创建一个equals方法.我可以通过做类似下面的事情(缩短一点)使这变得非常简单:

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

    if (obj == null)
        return false;

    if (getClass() != obj.getClass())
        return false;

    Address other = (Address) obj;

    return this.getStreet().equals(other.getStreet())
        && this.getStreetNumber().equals(other.getStreetNumber())
        && this.getStreetLetter().equals(other.getStreetLetter())
        && this.getTown().equals(other.getTown());
}

问题是,其中一些可能是空的.换句话说,NullPointerException如果此地址中没有街道信件,我会得到一个.

如何在考虑空值的情况下以干净的方式编写此文件？

Answer 1

您可以使用辅助方法

public static boolean isEqual(Object o1, Object o2) {
    return o1 == o2 || (o1 != null && o1.equals(o2));
}

Answer 2

您可以执行以下操作:

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

    if (obj == null) {
        return false;
    }

    if (getClass() != obj.getClass()) {
        return false;
    }

    Address other = (Address) obj;

    return equals(this.getStreet(),other.getStreet())
        && equals(this.getStreetNumber(), other.getStreetNumber())
        && equals(this.getStreetLetter(), other.getStreetLetter())
        && equals(this.getTown(), other.getTown());
}

private boolean equals(Object control, Object test) {
    if(null == control) {
        return null == test;
    }
    return control.equals(test);
}

Java 7引入了对java.util.Objects类的用例的内置支持,请参阅:

• java.utils.Objects.equals(Object,Object)
• java.utils.Objects.deepEquals(Object,Object)

Answer 3

Google Guava提供了Objects.equal(Object,Object),它检查相等性,同时考虑到其中任何一个参数可能为null:

...
return Objects.equal(this.getStreet(), other.getStreet())
    && Objects.equal(this.getStreetNumber(), other.getStreetNumber())
    && Objects.equal(this.getStreetLetter(), other.getStreetLetter())
    && Objects.equal(this.getTown(), other.getTown());

值得指出的是,Objects还有其他辅助方法来实现 hashCode()和toString().