Java学习笔记(八)——Lambda表达式

Lambda表达式

Lambda表达式是JDK8开始的一种新语法形式。

基本作用：简化函数式接口的匿名内部类的写法。

注意：

• Lambda表达式可以用来简化匿名内部类的书写
• Lambda表达式只能简化函数式接口的匿名内部类的写法

函数式接口：

有且仅有一个抽象方法的接口叫做函数式接口，接口上方可以加@FunctionalInterface注解

import java.util.Arrays;
import java.util.Comparator;public class test41 {public static void main(String[] args) {Integer[] integers = {5, 4, 8, 2, 4, 7, 6, 3, 0, 1, 9, 10};Arrays.sort(integers, (Integer o1, Integer o2) -> {return o1 - o2;});System.out.println(Arrays.toString(integers));  //[0, 1, 2, 3, 4, 4, 5, 6, 7, 8, 9, 10]}
}

Lambda表达式的省略写法

省略核心：可推导，可省略。

省略规则：

1.参数类型可以省略不写。

import java.util.Arrays;
import java.util.Comparator;public class test41 {public static void main(String[] args) {Integer[] integers = {5, 4, 8, 2, 4, 7, 6, 3, 0, 1, 9, 10};Arrays.sort(integers, (o1, o2) -> {return o1 - o2;});System.out.println(Arrays.toString(integers));  //[0, 1, 2, 3, 4, 4, 5, 6, 7, 8, 9, 10]}
}

2.如果只有一个参数，参数类型可以省略，同时()也可以省略。

3.如果Lambda表达式的方法体只有一行，大括号，分号，return可以省略不写，注意三者需要同时省略。

import java.util.Arrays;
import java.util.Comparator;public class test41 {public static void main(String[] args) {Integer[] integers = {5, 4, 8, 2, 4, 7, 6, 3, 0, 1, 9, 10};Arrays.sort(integers, (o1, o2) -> o1-o2);System.out.println(Arrays.toString(integers));  //[0, 1, 2, 3, 4, 4, 5, 6, 7, 8, 9, 10]}
}

Lambda练习

练习1

定义数组并存储一些字符串，按照字符串的长度进行排序，

短的在前，长的在后

import java.util.Arrays;
import java.util.Comparator;public class test42 {public static void main(String[] args) {String[] strings = {"cjm", "cjm_big_pig", "cjm_pig"};Arrays.sort(strings, (o1, o2) -> o1.length()-o2.length());System.out.println(Arrays.toString(strings));  //[cjm, cjm_pig, cjm_big_pig]}
}

练习2

定义数组并存储一些对象，

要求1：属性有姓名，年龄，身高

要求2：按照年龄的大小进行排序，年龄一样，按照身高排序，身高一样

按照姓名的字母进行排序（姓名中不要有中文或特殊字符）

public class pig {private String name;private int age;private int height;public pig() {}public pig(String name, int age, int height) {this.name = name;this.age = age;this.height = height;}public String getName() {return name;}public int getAge() {return age;}public int getHeight() {return height;}public void setName(String name) {this.name = name;}public void setAge(int age) {this.age = age;}public void setHeight(int height) {this.height = height;}}

import java.util.Arrays;
import java.util.Comparator;public class test43 {public static void main(String[] args) {pig pig1 = new pig("cjm", 29, 160);pig pig2 = new pig("djm", 25, 165);pig pig3 = new pig("djm", 25, 175);pig pig4 = new pig("cjm", 25, 175);pig[] pigs = {pig1, pig2, pig3, pig4};Arrays.sort(pigs, new Comparator<pig>() {@Overridepublic int compare(pig o1, pig o2) {if (o1.getAge() != o2.getAge()) {return o1.getAge() - o2.getAge();} else {if (o1.getHeight() != o2.getHeight()) {return o1.getHeight() - o2.getHeight();} else {return o1.getName().compareTo(String.valueOf(o2));}}}});for (pig pig : pigs) {System.out.println(pig.getName() + " " + pig.getAge() + " " + pig.getHeight());}}
}

运行结果：

djm 25 165
cjm 25 175
djm 25 175
cjm 29 160

算法题

算法题1 斐波那契数列

import java.util.ArrayList;public class test44 {public static void main(String[] args) {ArrayList<Integer> arrayList = new ArrayList<>();arrayList.add(1);arrayList.add(1);for (int i = 2; i < 20; i++) {arrayList.add(arrayList.get(i - 1) + arrayList.get(i - 2));}System.out.println(arrayList.get(11));  //144}
}

算法题2 猴子吃桃子

public class test45 {public static void main(String[] args) {int[] ints = new int[10];ints[9] = 1;for (int i = 8; i >= 0; i--) {ints[i] = (ints[i + 1] + 1) * 2;}System.out.println(ints[0]);  //1534}
}

public class test45 {public static void main(String[] args) {System.out.println(getCount(1));}public static int getCount(int day){if(day<=0||day>=11){System.out.println("时间错误！");return -1;}if(day==10){return 1;}return (getCount(day+1)+1)*2;}
}

算法题3 爬楼梯

public class test46 {public static void main(String[] args) {System.out.println(Fn(20));  //10946}public static int Fn(int n) {if (n == 1) {return 1;}if (n == 2) {return 2;}return Fn(n - 1) + Fn(n - 2);}
}

一次可以爬1个或2个或3个台阶：

public class test46 {public static void main(String[] args) {System.out.println(Fn(20));  //121415}public static int Fn(int n) {if (n == 1) {return 1;}if (n == 2) {return 2;}if (n == 3) {return 4;}return Fn(n - 1) + Fn(n - 2) + Fn(n - 3);}
}