Java中List.contains(Object object)方法怎麼使用

使用List.contains(Object object)方法判斷ArrayList是否包含一個元素物件（針對於物件的屬性值相同，但物件位址不同的情況），如果沒有重寫List1a4db2c2c2313771e5742b6debf617a1的元素物件Object中的equals方法，預設如下：

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

將導致contains方法始終傳回false。

查看ArrayList的contains方法的原始碼如下：

    /**
     * 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Object的equals方法來判斷是否包含某一元素，繼續查看Object類別中的equals方法，原始碼如下：

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

所以，使用「==」比較物件的位址，如果是同一物件即位址相同的情況下，才會傳回true，而對於物件屬性值相同但位址不同的不同對象，始終返回false！

如果需要依據物件屬性值是否相同來判斷ArrayList是否包含某一對象，則需要重寫Object的equals方法，並在equals方法中一一比較對象的每個屬性值，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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