Java集合和MapKey

Question

我是java的新手.我有一个数据库表,其中包含以下复合键:

Code
Reference_Number_1 (decimal)
Reference_Number_2 (decimal)
Time

上面的键使每行成为唯一的行 - 没有重复.我需要创建一个类来将此表加载到java集合并创建一个静态方法,该方法将以上四个参数作为键并从java集合返回实体.

我正在考虑将表加载到HashMap中,但我不确定如何定义MapKey.我应该将Reference_Number_1,Reference_Number_2和Time转换为字符串,然后连接这四个字段吗？或者还有另一种方法/集合来加载此表.谢谢,皮特

Answer 1

创建另一个类,它将这4个字段保存为属性并实现/自动生成equals()并hashCode() 根据合同(重要!否则它不能用作正确的Map密钥),最后将其用作(复合)密钥Map.

以下是Eclipse为我自动生成的内容(equals()开放性改进,有点冗长):

public class CompositeKey {

    private String code;
    private BigDecimal referenceNumber1;
    private BigDecimal referenceNumber2;
    private Date time;

    public CompositeKey(String code, BigDecimal referenceNumber1, BigDecimal referenceNumber2, Date time) {
        this.code = code;
        this.referenceNumber1 = referenceNumber1;
        this.referenceNumber2 = referenceNumber2;
        this.time = time;
    }

    public String getCode() {
        return code;
    }

    public BigDecimal getReferenceNumber1() {
        return referenceNumber1;
    }

    public BigDecimal getReferenceNumber2() {
        return referenceNumber2;
    }

    public Date getTime() {
        return time;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        CompositeKey other = (CompositeKey) obj;
        if (code == null) {
            if (other.code != null)
                return false;
        }
        else if (!code.equals(other.code))
            return false;
        if (referenceNumber1 == null) {
            if (other.referenceNumber1 != null)
                return false;
        }
        else if (!referenceNumber1.equals(other.referenceNumber1))
            return false;
        if (referenceNumber2 == null) {
            if (other.referenceNumber2 != null)
                return false;
        }
        else if (!referenceNumber2.equals(other.referenceNumber2))
            return false;
        if (time == null) {
            if (other.time != null)
                return false;
        }
        else if (!time.equals(other.time))
            return false;
        return true;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((code == null) ? 0 : code.hashCode());
        result = prime * result + ((referenceNumber1 == null) ? 0 : referenceNumber1.hashCode());
        result = prime * result + ((referenceNumber2 == null) ? 0 : referenceNumber2.hashCode());
        result = prime * result + ((time == null) ? 0 : time.hashCode());
        return result;
    }

}