从List <Foo>到Map <String,List <Foo >>:寻找更好的实现

Question

让我告诉你我的代码:

Foo类

public class Foo {

    String code;
    String value;

    public Foo(String code, String value) {
        super();
        this.code = code;
        this.value = value;
    }
    // getters/setters
}

主要方法(专注于getFooMultiMapCode()方法):

public class FooMain {

public static void main(String[] args) {
    Foo foo1 = new Foo("100","foo1");
    Foo foo2 = new Foo("200","foo2");
    Foo foo3 = new Foo("300","foo3");
    Foo foo4 = new Foo("100","foo4");
    Foo foo5 = new Foo("100","foo5");
    Foo foo6 = new Foo("200","foo6");
    List<Foo> foos = Arrays.asList(foo1,foo2,foo3,foo4,foo5,foo6);  
    Map<String,List<Foo>> fooCodeMap = getFooMultiMapCode(foos);
    System.out.println(fooCodeMap);
}

private static Map<String, List<Foo>> getFooMultiMapCode(List<Foo> foos) {
    Map<String, List<Foo>> fooMultiMapCode = new HashMap<String, List<Foo>>();
    for(Foo foo:foos){
        List<Foo> list = fooMultiMapCode.get(foo.getCode());
        if(list==null){
            list = new ArrayList<Foo>();
            list.add(foo);
            fooMultiMapCode.put(foo.getCode(), list);
        }
        else {
            list.add(foo);
        }
    }
    return fooMultiMapCode;
}
}

Main正确打印此字符串:

{100=[foo1, foo4, foo5], 200=[foo2, foo6], 300=[foo3]}

我想getFooMultiMapCode用更简洁的方式重写方法,例如使用java8或者像lambdaj,guava等库,但我不想改变方法签名.

Answer 1

Java 8的解决方案和groupingBy(classifier, downstream):

return foos.stream().collect(Collectors.groupingBy(Foo::getCode, Collectors.toList()));

或只是groupingBy(classifier)作为@Boris蜘蛛注意到:

return foos.stream().collect(Collectors.groupingBy(Foo::getCode));