HashMapのキー情報を後から参照するには？

public class SamplePojo {
    private String id;
    private Integer field;

    public SamplePojo() { }

    public SamplePojo(String id) {
        this.id = id;
    }

    public SamplePojo(String id, Integer field) {
        this.id = id;
        this.field = field;
    }

// 〜getter/setter省略〜

    public boolean equals(Object other) {
        if (this == other ) return true;
        if ( !(other instanceof SamplePojo) ) return false;
        SamplePojo castOther = (SamplePojo)other;
        return new EqualsBuilder()
            .append(this.getId(), castOther.getId())
            .isEquals();
    }

    public int hashCode() {
        return new HashCodeBuilder()
            .append(getId())
            .toHashCode();
    }

}

public class Test {
    public static void main(String[] args) {
        Map<SamplePojo, String> map = new HashMap<SamplePojo, String>();
        SamplePojo key = new SamplePojo("Pojo1", Integer.valueOf(0));
        map.put(key, "a");
        // 〜他にもmapにデータを格納する処理〜

        // 既存キーの検索・参照
        SamplePojo find = new SamplePojo("Pojo1");
        if (map.containsKey(find)) {
            System.out.println("実際のkeyのfieldは？");
        }
    }
}