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:
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@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:
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/** * 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:
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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:
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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 + "]" ; } } |
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原文链接:https://blog.csdn.net/growing_tree/article/details/46622579