List.contains(Object object)方法，比较对象是否相同

Posted 2021-02-27 亦枫Blog

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 + "]";
	}
	
}