本文主要介绍Java中,通过Stream collect()方法,指定条件获取找到List(列表)中最小的对象元素的方法。

class Foo {
    private int variableCount;
    public Foo(int vars) {
        this.variableCount = vars; 
    }
    public Integer getVariableCount() { 
      return variableCount; 
    }
}

1、实现accumulator object用于给Collector提供函数

/**
* Accumulator object to hold the current min
* and the list of Foos that are the min.
*/
class Accumulator {
Integer min;
List<Foo> foos;
Accumulator() {
min = Integer.MAX_VALUE;
foos = new ArrayList<>();
}
void accumulate(Foo f) {
if (f.getVariableCount() != null) {
if (f.getVariableCount() < min) {
min = f.getVariableCount();
foos.clear();
foos.add(f);
} else if (f.getVariableCount() == min) {
foos.add(f);
}
}
}
Accumulator combine(Accumulator other) {
if (min < other.min) {
return this;
}
else if (min > other.min) {
return other;
}
else {
foos.addAll(other.foos);
return this;
}
}
List<Foo> getFoos() { return foos; }
}

2、调用方法和测试

1)示例List<Foo>

List<Foo> foos = Arrays.asList(new Foo(3), new Foo(3), new Foo(2), new Foo(1), new Foo(1), new Foo(4));

2)调用方法

List<Foo> mins = foos.stream().collect(Collector.of(
    Accumulator::new,
    Accumulator::accumulate,
    Accumulator::combine,
    Accumulator::getFoos
    )
);

输出结果:

[Foo{1}, Foo{1}]