使用自定义密钥的Java收集分组

Question

我有一个学生的列表studentList，我想根据他们的班级和部分将它们分开，并将其分为map＆lt; customkey，list＆lt; student＆gt;＆gt;＆gt;。

class Student {
public String Name;
public String Class;
public String Section;
}

我有一个自定义的密钥类，

class CustomKey {
public String Class;
public String Section;
}

我正在尝试使用此功能将它们分组，但是我们可以清楚地看到我做错了什么。

studentList.stream().collect(Collectors.groupingBy(x -> new CustomKey(x.Class, x.Section)));

组内的表达式不正确。 不允许扩展学生班:(

我想创建此输出，

{
    {"ClassA", "SectionA"}: [{name, class, section}, {name, class, section}],
    {"ClassA", "SectionB"}: [{name, class, section}],
    {"ClassB", "SectionA"}: [{name, class, section}, {name, class, section}],
}

我的知识在Java上非常有限。任何帮助/指示都受到欢迎。 另外，对于混乱的案件表示歉意。

Answer 1

好吧，customKey与其equals方法匹配，而您没有被覆盖。如果您实现等于（以及hashcode），则它将起作用。

更好的是，为CustomKey创建 record ：

public record CustomKey(String className, String section) { }

这将使编译器自动生成其等于 equals 和hashcode < /code>免费实现，以及getters canonical构造函数和toString方法。上述记录定义等同于此：

public final class CustomKey {

    private final String className;
    private final String section;

    public CustomKey(String className, String section) {
        this.className = className;
        this.section = section;
    }

    public String className() {
        return className;
    }

    public String section() {
        return section;
    }

    @Override
    public boolean equals(Object o) {
        return o instanceof CustomKey ck
            && Objects.equals(className, ck.className)
            && Objects.equals(section, ck.section);
    }

    @Override
    public int hashCode() {
        return Objects.hash(className, section);
    }

    @Override
    public String toString() {
        return "CustomKey[className=" + className + ", section=" + section + "]";
    }
}

Answer 2

我完全像下面一样使用您的代码，它对我有用，

 public void testCustomGroupBy(){
    List<Student> studentList = new ArrayList<>();
    studentList.add(new Student("name_1","class_1","section_1"));
    studentList.add(new Student("name_2","class_1","section_1"));
    studentList.add(new Student("name_3","class_2","section_1"));
    studentList.add(new Student("name_4","class_2","section_2"));
    studentList.add(new Student("name_5","class_2","section_2"));
    Map<CustomKey, List<Student>> studentsBySection = studentList.stream().collect(Collectors.groupingBy(x -> new CustomKey(x.getClassName(), x.getSection())));
    studentsBySection.entrySet().forEach(System.out::print);
}

例如下面创建您的pojo

class Student {
public String name;
public String className;
public String section;
// getters and setters

public Student(String name, String className, String section) {
    this.name = name;
    this.className = className;
    this.section = section;
}

}}

class CustomKey {
public String className;
public String section;
// getters and setters

public CustomKey(String className, String section) {
    this.className = className;
    this.section = section;
}

}

output

CustomKey(className=class_2, section=section_2)=[Student(name=name_4, className=class_2, section=section_2), Student(name=name_5, className=class_2, section=section_2)]
CustomKey(className=class_2, section=section_1)=[Student(name=name_3, className=class_2, section=section_1)]
CustomKey(className=class_1, section=section_1)=[Student(name=name_1, className=class_1, section=section_1), Student(name=name_2, className=class_1, section=section_1)]