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 + "]";
}
}
