重寫Object類的hashCode和equals方法的一次嘗試

爲何重寫？

在項目開發中遇到須要將Object對象做爲HashMap的key使用的場景，爲了保證HashMap的key的惟一性，故對該對象進行了重寫hashCode方法和重寫equals方法。

代碼示例

package com.xxx.xxx;

/**
 * @Author: 
 * @Date: 
 * @Description:
 */
public class ColumnDTO {

    private String columnid;

    private String columnname;

    private String nodepath;

    private String upperid;

    private String linkurl;


    public String getColumnid() {
        return columnid;
    }

    public void setColumnid(String columnid) {
        this.columnid = columnid;
    }

    public String getColumnname() {
        return columnname;
    }

    public void setColumnname(String columnname) {
        this.columnname = columnname;
    }

    public String getNodepath() {
        return nodepath;
    }

    public void setNodepath(String nodepath) {
        this.nodepath = nodepath;
    }

    public String getUpperid() {
        return upperid;
    }

    public void setUpperid(String upperid) {
        this.upperid = upperid;
    }

    public String getLinkurl() {
        return linkurl;
    }

    public void setLinkurl(String linkurl) {
        this.linkurl = linkurl;
    }


    /**
     * 重寫hashCode方法和equals方法，保證該對象能夠看成HashMap的Key使用
     */

    @Override
    public int hashCode() {
        int result = 17;
        result = 31 * result + (columnid == null ? 0 : columnid.hashCode());
        result = 31 * result + (columnname == null ? 0 : columnname.hashCode());
        result = 31 * result + (nodepath == null ? 0 : nodepath.hashCode());
        result = 31 * result + (upperid == null ? 0 : upperid.hashCode());
        result = 31 * result + (linkurl == null ? 0 : linkurl.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if(this == obj){
            return true;//地址相等
        }

        if(obj == null){
            return false;//非空性：對於任意非空引用x，x.equals(null)應該返回false。
        }

        if(obj instanceof ColumnDTO){
            ColumnDTO other = (ColumnDTO) obj;
            //須要比較的字段相等，則這兩個對象相等
            if(this.columnid.equals(other.columnid)
                    && this.columnname.equals(other.columnname)
                    && this.nodepath.equals(other.nodepath)
                    && this.linkurl.equals(other.linkurl)
                    && this.upperid.equals(other.upperid)){
                return true;
            }
        }
        return false;
    }
}

突出重點及總結

@Override
    public int hashCode() {
        int result = 17;
        result = 31 * result + (columnid == null ? 0 : columnid.hashCode());
        result = 31 * result + (columnname == null ? 0 : columnname.hashCode());
        result = 31 * result + (nodepath == null ? 0 : nodepath.hashCode());
        result = 31 * result + (upperid == null ? 0 : upperid.hashCode());
        result = 31 * result + (linkurl == null ? 0 : linkurl.hashCode());
        return result;
    }

須要對每個成員變量進行相似的hashCode處理，將最終的結果返回做爲hashCode值。

@Override
    public boolean equals(Object obj) {
        if(this == obj){
            return true;//地址相等
        }

        if(obj == null){
            return false;//非空性：對於任意非空引用x，x.equals(null)應該返回false。
        }

        if(obj instanceof ColumnDTO){
            ColumnDTO other = (ColumnDTO) obj;
            //須要比較的字段相等，則這兩個對象相等
            if(this.columnid.equals(other.columnid)
                    && this.columnname.equals(other.columnname)
                    && this.nodepath.equals(other.nodepath)
                    && this.linkurl.equals(other.linkurl)
                    && this.upperid.equals(other.upperid)){
                return true;
            }
        }
        return false;
    }

須要對每個成員變量進行比較，全部成員變量都相同才返回true，不然返回false。