List.contains(Object object)方法

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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：

@Overridepublic boolean equals(Object o) {// TODO Auto-generated method stubreturn 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;}@Overridepublic 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);}@Overridepublic String toString() {return "QuestionInfo [questionId=" + questionId + ", answerId="+ answerId + ", subQuestionId=" + subQuestionId + ", result="+ result + "]";}}

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