List.contains(Object object)方法

List.contains(Object object)方法

使用List.contains(Object object)方法判斷ArrayList是否包含一個元素對象（針對於對象的屬性值相同，但對象地址不同的情況），如果沒有重寫List的元素對象Object中的equals方法，默認如下：

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

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Object的equals方法來判斷是否包含某一元素，繼續查看Object類中的equals方法，源碼如下：

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

如果需要依據對象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 + "]";
	}
	
}