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Java中List.contains(Object?object)方法使用

更新時(shí)間：2022年04月08日 11:21:20 作者：亦楓Blog

本文主要介紹了Java中List.contains(Object?object)方法，使用List.contains(Object?object)方法判斷ArrayList是否包含一個(gè)元素對(duì)象，感興趣的可以了解一下

使用List.contains(Object object)方法判斷ArrayList是否包含一個(gè)元素對(duì)象（針對(duì)于對(duì)象的屬性值相同，但對(duì)象地址不同的情況），如果沒(méi)有重寫(xiě)List<E>的元素對(duì)象Object中的equals方法，默認(rèn)如下：

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

將導(dǎo)致contains方法始終返回false。

查看ArrayList的contains方法的源碼如下：

    /**
     * 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方法依據(jù)Object的equals方法來(lái)判斷是否包含某一元素，繼續(xù)查看Object類(lèi)中的equals方法，源碼如下：

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

所以，使用“==”比較對(duì)象的地址，如果是同一對(duì)象即地址相同的情況下，才會(huì)返回true，而對(duì)于對(duì)象屬性值相同但地址不同的不同對(duì)象，始終返回false！

如果需要依據(jù)對(duì)象屬性值是否相同來(lái)判斷ArrayList是否包含某一對(duì)象，則需要重寫(xiě)Object的equals方法，并在equals方法中一一比較對(duì)象的每個(gè)屬性值，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 + "]";
?? ?}
}

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