Java中Stream流

Java中Stream流的相关练习

1、练习一

import java.util.Arrays;
import java.util.List;
import java.util.Optional;

import org.junit.Test;

import com.atguigu.java8.Employee;
import com.atguigu.java8.Employee.Status;

public class TestStreamAPI {

    /*
	  	1.	给定一个数字列表，如何返回一个由每个数的平方构成的列表呢？
		，给定【1，2，3，4，5】， 应该返回【1，4，9，16，25】。
	 */
    @Test
    public void test1(){
        Integer[] nums = new Integer[]{1,2,3,4,5};

        Arrays.stream(nums)
            .map((x) -> x * x)
            .forEach(System.out::println);
    }

    /*
	 2.	怎样用 map 和 reduce 方法数一数流中有多少个Employee呢？
	 */
    List<Employee> emps = Arrays.asList(
        new Employee(102, "李四", 59, 6666.66, Status.BUSY),
        new Employee(101, "张三", 18, 9999.99, Status.FREE),
        new Employee(103, "王五", 28, 3333.33, Status.VOCATION),
        new Employee(104, "赵六", 8, 7777.77, Status.BUSY),
        new Employee(104, "赵六", 8, 7777.77, Status.FREE),
        new Employee(104, "赵六", 8, 7777.77, Status.FREE),
        new Employee(105, "田七", 38, 5555.55, Status.BUSY)
    );

    @Test
    public void test2(){
        int sum = Integer.sum(2, 3);
        Optional<Integer> count = emps.stream()
            .map((e) -> 1)
            .reduce(Integer::sum);
        System.out.println(count.get());
        System.out.println(sum);
    }
}

结果依次展示：

2、练习二

交易员类
public class Trader {

	private String name;
	private String city;

	public Trader() {
	}

	public Trader(String name, String city) {
		this.name = name;
		this.city = city;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getCity() {
		return city;
	}

	public void setCity(String city) {
		this.city = city;
	}

	@Override
	public String toString() {
		return "Trader [name=" + name + ", city=" + city + "]";
	}
}

交易类
public class Transaction {

	private Trader trader;
	private int year;
	private int value;

	public Transaction() {
	}

	public Transaction(Trader trader, int year, int value) {
		this.trader = trader;
		this.year = year;
		this.value = value;
	}

	public Trader getTrader() {
		return trader;
	}

	public void setTrader(Trader trader) {
		this.trader = trader;
	}

	public int getYear() {
		return year;
	}

	public void setYear(int year) {
		this.year = year;
	}

	public int getValue() {
		return value;
	}

	public void setValue(int value) {
		this.value = value;
	}

	@Override
	public String toString() {
		return "Transaction [trader=" + trader + ", year=" + year + ", value="
				+ value + "]";
	}

}

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Stream;

import org.junit.Before;
import org.junit.Test;

public class TestTransaction {
	
	List<Transaction> transactions = null;
	
	@Before
	public void before(){
		Trader raoul = new Trader("Raoul", "Cambridge");
		Trader mario = new Trader("Mario", "Milan");
		Trader alan = new Trader("Alan", "Cambridge");
		Trader brian = new Trader("Brian", "Cambridge");
		
		transactions = Arrays.asList(
				new Transaction(brian, 2011, 300),
				new Transaction(raoul, 2012, 1000),
				new Transaction(raoul, 2011, 400),
				new Transaction(mario, 2012, 710),
				new Transaction(mario, 2012, 700),
				new Transaction(alan, 2012, 950)
		);
	}
	
	//1. 找出2011年发生的所有交易， 并按交易额排序（从低到高）
	@Test
	public void test1(){
		transactions.stream()
					.filter((t) -> t.getYear() == 2011)
					.sorted((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()))
					.forEach(System.out::println);
	}
	
	//2. 交易员都在哪些不同的城市工作过？
	@Test
	public void test2(){
		transactions.stream()
					.map((t) -> t.getTrader().getCity())
					.distinct()
					.forEach(System.out::println);
	}
	
	//3. 查找所有来自剑桥的交易员，并按姓名排序
	@Test
	public void test3(){
		transactions.stream()
					.filter((t) -> t.getTrader().getCity().equals("Cambridge"))
					.map(Transaction::getTrader)
					.sorted((t1, t2) -> t1.getName().compareTo(t2.getName()))
					.distinct()
					.forEach(System.out::println);
	}
	
	//4. 返回所有交易员的姓名字符串，按字母顺序排序
	@Test
	public void test4(){
		transactions.stream()
					.map((t) -> t.getTrader().getName())
					.sorted()
					.forEach(System.out::println);
		
		System.out.println("-----------------------------------");
		
		String str = transactions.stream()
					.map((t) -> t.getTrader().getName())
					.sorted()
					.reduce("", String::concat);
		
		System.out.println(str);
		
		System.out.println("------------------------------------");
		
		transactions.stream()
					.map((t) -> t.getTrader().getName())
					.flatMap(TestTransaction::filterCharacter)
					.sorted((s1, s2) -> s1.compareToIgnoreCase(s2))
					.forEach(System.out::print);
	}
	
	public static Stream<String> filterCharacter(String str){
		List<String> list = new ArrayList<>();
		
		for (Character ch : str.toCharArray()) {
			list.add(ch.toString());
		}
		
		return list.stream();
	}
	
	//5. 有没有交易员是在米兰工作的？
	@Test
	public void test5(){
		boolean bl = transactions.stream()
					.anyMatch((t) -> t.getTrader().getCity().equals("Milan"));
		
		System.out.println(bl);
	}
	
	
	//6. 打印生活在剑桥的交易员的所有交易额
	@Test
	public void test6(){
		Optional<Integer> sum = transactions.stream()
					.filter((e) -> e.getTrader().getCity().equals("Cambridge"))
					.map(Transaction::getValue)
					.reduce(Integer::sum);
		
		System.out.println(sum.get());
	}
	
	
	//7. 所有交易中，最高的交易额是多少
	@Test
	public void test7(){
		Optional<Integer> max = transactions.stream()
					.map((t) -> t.getValue())
					.max(Integer::compare);
		
		System.out.println(max.get());
	}
	
	//8. 找到交易额最小的交易
	@Test
	public void test8(){
		Optional<Transaction> op = transactions.stream()
					.min((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()));
		
		System.out.println(op.get());
	}

}

运行结果依次如下：