以下工具类方法基于Guava
List<Double>包装类型求值

• 求最大值

List<Double> list = ...;
Double max = Doubles.max(list.stream().mapToDouble(Double::valueOf).toArray());
Double max = Doubles.max(Doubles.toArray(list));

• 求最小值

List<Double> list = ..;
Double min = Doubles.min(list.stream().mapToDouble(Double::valueOf).toArray());
Double min = Doubles.min(Doubles.toArray(list));

• 求平均值

List<Double> list = ...;
Double average = list.stream().mapToDouble(Double::valueOf).average().getAsDouble();

List<User>对象类型求值

• 求User分数最大值

List<User> list = ...;
Double max = list.stream().mapToDouble(User::getScore).max().getAsDouble();

• 求User分数最小值

List<User> list = ...;
Double min= list.stream().mapToDouble(User::getScore).min().getAsDouble();

• 求User分数平均

List<User> list = ...;
Double average= list.stream().mapToDouble(User::getScore).average().getAsDouble();

其他用法

• 求List<Double>包装类型的中值

List<Double> list = ...;
// 先按升序排列集合
List<Double> transformList = Lists.newArrayList(list);
Ordering.usingToString();
Collections.sort(transformList);
//  取中值，list为偶数，取后面一个
Double middleValue = transformList.get(transformList.size()/2);

• 四舍五入，保留3位小数

double value = 1.23456;
double result = new BigDecimal(value).setScale(3, BigDecimal.ROUND_HALF_UP).doubleValue();
输出：1.235