背景

1. JDK1.8实现List中的数据分类。
2. 在JDK1.8之前是没有提供这样的API的，实现过程如下：JDK1.8之前实现List中的数据分类或者使用guava实现List分类数据

过程

1. 场景：需要把考生按照考生所属机构进行分类处理。已知考生在一个List容器中，并且考生有所属机构。
2. Candidate类

public class Candidate {
    private String orgId;//我的目的是以这个orgId分类
    private String orgName;
    private String candidateId;

    public Candidate(){

    }

    public Candidate(String orgId, String candidateId, String orgName) {
        super();
        this.candidateId = candidateId;
        this.orgId = orgId;
        this.orgName = orgName;
    }


    public String getCandidateId() {
        return candidateId;
    }

    public void setCandidateId(String candidateId) {
        this.candidateId = candidateId;
    }

    public String getOrgId() {
        return orgId;
    }

    public void setOrgId(String orgId) {
        this.orgId = orgId;
    }

    public String getOrgName() {
        return orgName;
    }

    public void setOrgName(String orgName) {
        this.orgName = orgName;

    }
}

3. CandidateOrg类

public class CandidateOrg {
    private String orgId;
    private String orgName;

    public CandidateOrg() {

    }

    public CandidateOrg(String orgId, String orgName) {
        super();
        this.orgId = orgId;
        this.orgName = orgName;
    }

    public String getOrgId() {
        return orgId;
    }

    public void setOrgId(String orgId) {
        this.orgId = orgId;
    }

    public String getOrgName() {
        return orgName;
    }

    public void setOrgName(String orgName) {
        this.orgName = orgName;
    }
}

3. 测试类

public class ClassificationTest {
    public static void main(String[] args) {
        List<Candidate> list = new ArrayList<Candidate>();
        list.add(new Candidate("1","1","机构一"));//机构id,考生id，机构名称。
        list.add(new Candidate("1","2","机构一"));
        list.add(new Candidate("2","3","机构二"));
        list.add(new Candidate("2","4","机构二"));
        list.add(new Candidate("3","5","机构三"));
        list.add(new Candidate("3","6","机构三"));
        list.add(new Candidate("4","7","机构四"));
        list.add(new Candidate("4","8","机构四"));

        Map<String, List<Candidate>> map = list.stream().collect(Collectors.groupingBy(
                Candidate::getOrgId));

    }
}

4. 结果（内存快照）
   

小结

1. 在JDK1.8之前确实没有找到一个好的实现方式。