1. List Stream distinct() 去重

List Stream 对象调用distinct()方法，distinct()方法依赖hashCode()和equals()方法。
判断两个对象是否相同原理与HashMap定位key原理相同，先计算hashCode，如果hashCode相同继续调用equals()方法

例子1

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class Jdk8TestNew {public static void main(String[] args) {List<String> list = Arrays.asList("AA", "BB", "CC", "BB", "CC", "AA", "AA");long l = list.stream().distinct().count();System.out.println("No. of distinct elements:"+l);String output = list.stream().distinct().collect(Collectors.joining(","));System.out.println(output);}}

结果

No. of distinct elements:3
AA,BB,CC

例子2

加一下User class

public class User {private String username;private String password;public User(String username, String password) {super();this.username = username;this.password = password;}@Overridepublic int hashCode() {System.out.println(this+", hashCode method");final int prime = 31;int result = 1;result = prime * result + ((password == null) ? 0 : password.hashCode());result = prime * result + ((username == null) ? 0 : username.hashCode());return result;}@Overridepublic boolean equals(Object obj) {System.out.println(obj+", equals method");if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;User other = (User) obj;if (password == null) {if (other.password != null)return false;} else if (!password.equals(other.password))return false;if (username == null) {if (other.username != null)return false;} else if (!username.equals(other.username))return false;return true;}@Overridepublic String toString() {return "User [username=" + username + ", password=" + password + "]";}}

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class Jdk8TestNew {public static void main(String[] args) {List<User> users = new ArrayList<>();users.add(new User("wangwang", "3"));users.add(new User("wangwang", "3"));users.add(new User("guagua", "2"));System.out.println(users.stream().distinct().collect(Collectors.toList()));}}

结果

User [username=wangwang, password=3], hashCode method
User [username=wangwang, password=3], hashCode method
User [username=wangwang, password=3], hashCode method
User [username=wangwang, password=3], equals method
User [username=guagua, password=2], hashCode method
User [username=guagua, password=2], hashCode method
[User [username=wangwang, password=3], User [username=guagua, password=2]]

2 Streams filter() and collect()

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;public class Jdk8TestNew {public static void main(String args[]) {List<String> lines = Arrays.asList("spring", "node", "mkyong");List<String> result0 = getFilterOutput(lines, "mkyong");// output "spring", "node"for (String temp : result0) {System.out.println(temp);}/* The equivalent example in Java 8, using stream.filter() tofilter a list, and collect() to convert a stream.*/List<String> result1 = lines.stream()  // convert list to stream.filter(line -> !"mkyong".equals(line)) // filter the line which equals to "mkyong".collect(Collectors.toList());  // collect the output and convert streams to a listresult1.forEach(System.out::println); // output "spring", "node"}private static List<String> getFilterOutput(List<String> lines, String filter) {List<String> result = new ArrayList<>();for (String line : lines) {if (!"mkyong".equals(line)) {result.add(line);}}return result;}}

结果

spring
node
spring
node

3 Try后面跟括号

java7新特性，支持使用try后面跟（）括号，用来管理释放资源，try括号内的资源会在try语句结束后自动释放，前提是这些课关闭的资源必须实现 java.lang.AutoCloseable接口

package com.download;import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;public class WordCount {public static void main(String[] args) {try (InputStream in = new FileInputStream("");OutputStream out = new FileOutputStream("");){byte[] buf = new byte[8192];int i;while ((i = in.read(buf)) != -1) {out.write(buf, 0, i);}} catch (IOException e) {e.printStackTrace();}}
}

4 JDK8中有双冒号的用法

JDK8中有双冒号的用法，就是把方法当做参数传到stream内部，使stream的每个元素都传入到该方法里面执行一下。

代码

public class AcceptMethod {public static void  printValur(String str){System.out.println("print value : "+str);}public static void main(String[] args) {List<String> al = Arrays.asList("a","b","c","d");for (String a: al) {AcceptMethod.printValur(a);}//下面的for each循环和上面的循环是等价的 al.forEach(x->{AcceptMethod.printValur(x);});}
}

现在JDK双冒号是：

public class MyTest {public static void  printValur(String str){System.out.println("print value : "+str);}public static void main(String[] args) {List<String> al = Arrays.asList("a", "b", "c", "d");al.forEach(AcceptMethod::printValur);//下面的方法和上面等价的Consumer<String> methodParam = AcceptMethod::printValur; //方法参数al.forEach(x -> methodParam.accept(x));//方法执行accept}
}

上面的所有方法执行玩的结果都是如下：

print value : a
print value : b
print value : c
print value : d

　在JDK8中，接口Iterable 8中默认实现了forEach方法，调用了 JDK8中增加的接口Consumer内的accept方法，执行传入的方法参数。

JDK源码如下：

/*** Performs the given action for each element of the {@code Iterable}* until all elements have been processed or the action throws an* exception.  Unless otherwise specified by the implementing class,* actions are performed in the order of iteration (if an iteration order* is specified).  Exceptions thrown by the action are relayed to the* caller.** @implSpec* <p>The default implementation behaves as if:* <pre>{@code*     for (T t : this)*         action.accept(t);* }</pre>** @param action The action to be performed for each element* @throws NullPointerException if the specified action is null* @since 1.8*/default void forEach(Consumer<? super T> action) {Objects.requireNonNull(action);for (T t : this) {action.accept(t);}}

另外补充一下，JDK8改动的，在接口里面可以有默认实现，就是在接口前加上default，实现这个接口的函数对于默认实现的方法可以不用再实现了。类似的还有static方法。现在这种接口除了上面提到的，还有BiConsumer,BiFunction,BinaryOperation等，在java.util.function包下的接口，大多数都有，后缀为Supplier的接口没有和别的少数接口。

5 lambda 的性能比较

public class AcceptMethod {public static void printValur(String str) {str = str.intern();CharSequence s = str.subSequence(0, 1);}public static void test1() {List<String> al = new ArrayList<String>();for (int i = 0; i < 10000; i++) {al.add("al" + i);}Long startTime = System.currentTimeMillis();for (String a : al) {AcceptMethod.printValur(a);}System.out.println("old:" + (System.currentTimeMillis() - startTime));startTime = System.currentTimeMillis();al.forEach(x -> {AcceptMethod.printValur(x);});System.out.println("new:" + (System.currentTimeMillis() - startTime));startTime = System.currentTimeMillis();al.forEach(AcceptMethod::printValur);System.out.println("new1:" + (System.currentTimeMillis() - startTime));}public static void test2() {HashMap<String, String> al = new HashMap<>();for (int i = 0; i < 10000; i++) {al.put(i+"", i+"");}Long startTime = System.currentTimeMillis();Set<Map.Entry<String, String>> ss = al.entrySet();for (Map.Entry<String, String> entry : ss) {AcceptMethod.printValur(entry.getKey());}System.out.println("old:" + (System.currentTimeMillis() - startTime));startTime = System.currentTimeMillis();al.forEach((x, y) -> {AcceptMethod.printValur(x);});System.out.println("new:" + (System.currentTimeMillis() - startTime));startTime = System.currentTimeMillis();al.forEach(new BiConsumer<String, String>(){@Overridepublic void accept(String t, String u) {AcceptMethod.printValur(t);}});System.out.println("new1:" + (System.currentTimeMillis() - startTime));}public static void test3() {Set<String> al = new HashSet<String>();for (int i = 0; i < 10000; i++) {al.add("al" + i);}Long startTime = System.currentTimeMillis();for (String a : al) {AcceptMethod.printValur(a);}System.out.println("old:" + (System.currentTimeMillis() - startTime));startTime = System.currentTimeMillis();al.forEach(x -> {AcceptMethod.printValur(x);});System.out.println("new:" + (System.currentTimeMillis() - startTime));startTime = System.currentTimeMillis();al.forEach(AcceptMethod::printValur);System.out.println("new1:" + (System.currentTimeMillis() - startTime));}public static void main(String[] args) {AcceptMethod.test2();
//      AcceptMethod.test2();
//      AcceptMethod.test3();}
}

执行完第一个test1()

old:3
new:30
new1:2

可以看到下面这种写法最慢了。所以不是所有JDK8的特性都是速度变快的。

al.forEach(x -> {
AcceptMethod.printValur(x);
});

执行完第一个test2()

old:3
new:31
new1:2

执行完第一个test3()

old:3
new:32
new1:2

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