list集合根据对象某属性求和，最大值等

先初始化集合并添加数据

//实体类
public class Student {
    private int mathScoresInt; //数学成绩
    private long mathScoresLong;//数学成绩
    private float mathScoresFloat;//数学成绩
    private double mathScoresDouble;//数学成绩
    private BigDecimal mathScoresBigDecimal;//数学成绩
    //构造方法忽略
    //set、get 方法忽略
}
//测试数据   ( 不允许list中存在为空的值，不然会异常! )
List<Student> list = new ArrayList();
list.add(new Student(87, 87, 87.5f, 87.8, new BigDecimal(87)));
list.add(new Student(88, 88, 88.5f, 88.8, new BigDecimal(88)));
list.add(new Student(89, 89, 89.5f, 89.8, new BigDecimal(89)));
list.add(new Student(90, 90, 90.5f, 90.8, new BigDecimal(90)));

提示：以下计算为int 、long 、float 、double 、BigDecimal 等类型

一、根据List中的对象的某个属性，求和

double mathAverageInt = list.stream().mapToInt( Student::getMathScoresInt ).average().orElse(0d);
double mathAverageLong = list.stream().mapToLong( Student::getMathScoresLong ).average().orElse(0d);
double mathAverageDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).average().orElse(0d);
二、根据List中的对象的某个属性，求平均值

double mathAverageInt = list.stream().mapToInt( Student::getMathScoresInt ).average().orElse(0d);
double mathAverageLong = list.stream().mapToLong( Student::getMathScoresLong ).average().orElse(0d);
double mathAverageDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).average().orElse(0d);
三、根据List中的对象的某个属性，求最大值

int mathMaxInt = list.stream().mapToInt( Student::getMathScoresInt ).max().getAsInt(); //int类型
long mathMaxLong = list.stream().mapToLong( Student::getMathScoresLong ).max().getAsLong();
double mathMaxDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).max().getAsDouble();
BigDecimal mathMaxBigDecimal = list.stream().map( Student::getMathScoresBigDecimal ).reduce(BigDecimal.ZERO, BigDecimal::max);
四、根据List中的对象的某个属性，求最小值

int mathMinInt = list.stream().mapToInt( Student::getMathScoresInt ).min().getAsInt();
long mathMinLong = list.stream().mapToLong( Student::getMathScoresLong ).min().getAsLong();
double mathMinDouble = list.stream().mapToDouble( Student::getMathScoresDouble ).min().getAsDouble();
BigDecimal mathMinBigDecimal = list.stream().map( Student::getMathScoresBigDecimal ).reduce(BigDecimal.ZERO, BigDecimal::min);