Java. ArrayList的包含方法不起作用 [英] Java. ArrayList's contains method not working

问题描述

我正在检查ArrayList是否包含对象:

I am checking if ArrayList contains object:

            List<Property> propertiesByName = getPropertiesByCategory(propertyCategory);
        for(Property property: propertiesByName){
            List<Property> propertyList = getVariationItem().getProperties();
            Galgo.log("*******************");
            Galgo.log(propertyList.toString());
            Galgo.log(property.toString());
            Galgo.log("contains:"+propertyList.contains(property));
        }

我正在获取以下日志:

*******************
[Property{name='color', value='red'}, Property{name='size', value='42'}]
Property{name='color', value='red'}
contains:false
*******************
[Property{name='color', value='red'}, Property{name='size', value='42'}]
Property{name='color', value='blue'}
contains:false
Database: get 2 variations
*******************
[Property{name='color', value='red'}, Property{name='size', value='42'}]
Property{name='size', value='42'}
contains:false
*******************
[Property{name='color', value='red'}, Property{name='size', value='42'}]
Property{name='size', value='34'}
contains:false

如您在第一种和第三种情况中所看到的，它应该返回true.怎么了?

As you can see in the first and third cases, it should return true. What is wrong?

我的代码的其他部分.按类别(颜色，大小)获取属性的第一种方法.第二种方法是获取所有可用属性:

Other parts of my code. First method to get properties by category(color, size). Second method is to get all available properties:

            private List<Property> getPropertiesByCategory(String category){
            List<Property> properties = new ArrayList<>();
            for(Property property: getAllProperties()){
                if(property.getName().equals(category)){
                    if(!properties.contains(property)){
                        properties.add(property);
                    }
                }
            }
            return properties;
        }

        private List<Property> getAllProperties() {
            List<Property> propertyList = new ArrayList<>();
            for(VariationItem variationItem: getProductItem().getVariationsList()){
                for(Property property: variationItem.getProperties()){
                    if(!propertyList.contains(property))
                    {
                        propertyList.add(property);
                    }
                }
            }
            return propertyList;
        }

推荐答案

要使用contains方法，必须重写equals()和hashCode()方法以实现此目的.您可以在 https://stackoverflow.com/a/16069158/1320616 上查看此答案.实际上，contains()将比较两个对象.并且要比较两个对象，您必须实现equals()方法.

To use contains method, you have to override equals() and hashCode() methods to achieve this. You can check this answer for implementation https://stackoverflow.com/a/16069158/1320616. Actually contains() will compare two objects. And to compare two objects you have to implement equals() method.

这里是完整的详细信息

因此，当您使用contains()时，它的作用就是

So when you use contains() what it is doing is

@Override public boolean contains(Object object) {
        Object[] a = array;
        int s = size;
        if (object != null) {
            for (int i = 0; i < s; i++) {
                if (object.equals(a[i])) {
                    return true;
                }
            }
        } else {
            for (int i = 0; i < s; i++) {
                if (a[i] == null) {
                    return true;
                }
            }
        }
        return false;
    }

，如果您未在类中实现equals()方法，它将从Object类(即

and if you don't implement equals() method in your class it will take the equals() method from the Object class which is

public boolean equals(Object o) {
        return this == o;
}

因此现在减少到==正在两个对象之间进行比较.当在两个对象之间放置==时，它将基于两件事进行比较(a)两个对象的哈希码(b)使用两个对象的toString().

So now it reduces to == is being used between two objects to compare them. When you put == between two objects, it compares on basis of two things (a) hashcodes of two objects (b) It uses toString() of the two objects.

每个对象都分配有不同的哈希码.这就是您的contains()无法为您提供正确结果的原因.

Each object has different hashcodes assigned to them. this is the reason why your contains() is not giving you correct results.

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