List.contains(Object object)方法，比较对象是否相同

原文地址：https://blog.csdn.net/growing_tree/article/details/46622579

使用List.contains(Object object)方法判断ArrayList是否包含一个元素对象（针对于对象的属性值相同，但对象地址不同的情况），如果没有重写List<E>的元素对象Object中的equals方法，默认如下：

@Override
public boolean equals(Object o) {
    // TODO Auto-generated method stub
    return super.equals(o);
}

将导致contains方法始终返回false。

查看ArrayList的contains方法的源码如下：

/**
 * Searches this {@code ArrayList} for the specified object.
 *
 * @param object
 *            the object to search for.
 * @return {@code true} if {@code object} is an element of this
 *         {@code ArrayList}, {@code false} otherwise
 */
@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;
}

可以看出，contains方法依据Object的equals方法来判断是否包含某一元素，继续查看Object类中的equals方法，源码如下：

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

所以，使用“==”比较对象的地址，如果是同一对象即地址相同的情况下，才会返回true，而对于对象属性值相同但地址不同的不同对象，始终返回false！

如果需要依据对象属性值是否相同来判断ArrayList是否包含某一对象，则需要重写Object的equals方法，并在equals方法中一一比较对象的每个属性值，如：

package com.feng.lejuan.entity;

public class QuestionInfo {

    private String questionId;

    private String answerId;

    private String subQuestionId;

    private String result;

    public QuestionInfo() {
        super();

    }

    public QuestionInfo(String questionId, String answerId,
            String subQuestionId, String result) {
        super();
        this.questionId = questionId;
        this.answerId = answerId;
        this.subQuestionId = subQuestionId;
        this.result = result;
    }

    public String getQuestionId() {
        return questionId;
    }

    public void setQuestionId(String questionId) {
        this.questionId = questionId;
    }

    public String getAnswerId() {
        return answerId;
    }

    public void setAnswerId(String answerId) {
        this.answerId = answerId;
    }

    public String getSubQuestionId() {
        return subQuestionId;
    }

    public void setSubQuestionId(String subQuestionId) {
        this.subQuestionId = subQuestionId;
    }

    public String getResult() {
        return result;
    }

    public void setResult(String result) {
        this.result = result;
    }

    @Override
    public boolean equals(Object o) {
        if (o instanceof QuestionInfo) {
            QuestionInfo question = (QuestionInfo) o;
            return this.questionId.equals(question.questionId)
                    && this.subQuestionId.equals(question.subQuestionId)
                    && this.answerId.equals(question.answerId)
                    && this.result.equals(question.result);
        }
        return super.equals(o);
    }

    @Override
    public String toString() {
        return "QuestionInfo [questionId=" + questionId + ", answerId="
                + answerId + ", subQuestionId=" + subQuestionId + ", result="
                + result + "]";
    }

}