能夠使用Lambda的依據是必須有相應的函數接口(函數接口,是指內部只有一個抽象方法的接口)。這一點跟Java是強類型語言吻合,也就是說你並不能在代碼的任何地方任性的寫Lambda表達式。實際上Lambda的類型就是對應函數接口的類型。Lambda表達式另一個依據是類型推斷機制(重點),在上下文信息足夠的情況下,編譯器可以推斷出參數表的類型,而不需要顯式指名。
一、演化過程
A.基礎類的準備
package com.os.model;
import java.util.Objects;
public class Employee {
private int id;
private String name;
private int age;
private double salary;
//省略生成的getteer和setter方法,構造方法、toString方法、hashCode、equals方法
}
B.篩選數據
我們根據不同的篩選條件需要設置不同的方法,增加了很多的代碼量。
package com.os.test;
import com.os.model.Employee;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Demo01 {
private static List<Employee> emps = Arrays.asList(
new Employee(101, "悟空", 18, 9999.99),
new Employee(102, "八戒", 59, 6666.66),
new Employee(103, "唐僧", 28, 3333.33),
new Employee(104, "沙僧", 8, 7777.77),
new Employee(105, "白龍馬", 38, 5555.55)
);
public static void main(String[] args) {
System.out.println("===>1.篩選年齡");
List<Employee> list = filterEmployeeAge(emps);
for (Employee employee : list) {
System.out.println(employee);
}
System.out.println("===>2.篩選工資");
list = filterEmployeeSalary(emps);
for (Employee employee : list) {
System.out.println(employee);
}
}
/**
* 需求:獲取公司中年齡小於 35 的員工信息
* @param employeeList
* @return
*/
public static List<Employee> filterEmployeeAge(List<Employee> employeeList){
List<Employee> list = new ArrayList<>();
for (Employee emp : employeeList) {
if(emp.getAge() <= 35){
list.add(emp);
}
}
return list;
}
/**
* 需求:獲取公司中工資大於 5000 的員工信息
* @param employeeList
* @return
*/
public static List<Employee> filterEmployeeSalary(List<Employee> employeeList){
List<Employee> list = new ArrayList<>();
for (Employee emp : employeeList) {
if(emp.getSalary() >= 5000){
list.add(emp);
}
}
return list;
}
}
C.代碼進化:策略設計模式
(1)定義篩選條件的泛型接口
package com.os.service;
/**
* 針對於數據的篩選條件的接口
*/
public interface ObjectDataPredicate<T> {
boolean test(T t);
}
(2)實現類去實現不同的篩選方式
按照年齡進行篩選實現類
package com.os.service;
import com.os.model.Employee;
public class FilterEmployeeForAge implements ObjectDataPredicate<Employee> {
@Override
public boolean test(Employee employee) {
return employee.getAge() <= 35;
}
}
按照工資進行篩選實現類
package com.os.service;
import com.os.model.Employee;
public class FilterEmployeeForSalary implements ObjectDataPredicate<Employee> {
@Override
public boolean test(Employee employee) {
return employee.getSalary() >= 5000;
}
}
(3)策略模式的實現代碼
public static List<Employee> filterEmployee(List<Employee> emps, ObjectDataPredicate<Employee> objectDataPredicate){
List<Employee> list = new ArrayList<>();
for (Employee employee : emps) {
if(objectDataPredicate.test(employee)){
list.add(employee);
}
}
return list;
}
完整代碼如下
package com.os.test;
import com.os.model.Employee;
import com.os.service.FilterEmployeeForAge;
import com.os.service.FilterEmployeeForSalary;
import com.os.service.ObjectDataPredicate;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Demo02 {
private static List<Employee> emps = Arrays.asList(
new Employee(101, "悟空", 18, 9999.99),
new Employee(102, "八戒", 59, 6666.66),
new Employee(103, "唐僧", 28, 3333.33),
new Employee(104, "沙僧", 8, 7777.77),
new Employee(105, "白龍馬", 38, 5555.55)
);
public static void main(String[] args) {
List<Employee> list = filterEmployee(emps, new FilterEmployeeForAge());//接口回調
for (Employee employee : list) {
System.out.println(employee);
}
System.out.println("------------------------------------------");
List<Employee> list2 = filterEmployee(emps, new FilterEmployeeForSalary());//接口回調
for (Employee employee : list2) {
System.out.println(employee);
}
}
public static List<Employee> filterEmployee(List<Employee> emps, ObjectDataPredicate<Employee> objectDataPredicate){
List<Employee> list = new ArrayList<>();
for (Employee employee : emps) {
if(objectDataPredicate.test(employee)){
list.add(employee);
}
}
return list;
}
}
這種代碼的實現類太多了
D.代碼進化:匿名內部類
package com.os.test;
import com.os.model.Employee;
import com.os.service.FilterEmployeeForAge;
import com.os.service.FilterEmployeeForSalary;
import com.os.service.ObjectDataPredicate;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Demo03 {
private static List<Employee> emps = Arrays.asList(
new Employee(101, "悟空", 18, 9999.99),
new Employee(102, "八戒", 59, 6666.66),
new Employee(103, "唐僧", 28, 3333.33),
new Employee(104, "沙僧", 8, 7777.77),
new Employee(105, "白龍馬", 38, 5555.55)
);
public static void main(String[] args) {
List<Employee> list = filterEmployee(emps, new ObjectDataPredicate<Employee>() {
@Override
public boolean test(Employee employee) {
return employee.getId() <= 103;
}
});//接口回調
for (Employee employee : list) {
System.out.println(employee);
}
}
public static List<Employee> filterEmployee(List<Employee> emps, ObjectDataPredicate<Employee> objectDataPredicate){
List<Employee> list = new ArrayList<>();
for (Employee employee : emps) {
if(objectDataPredicate.test(employee)){
list.add(employee);
}
}
return list;
}
}
通過內部類,我們能發現整個代碼中核心的部分就是一句話
employee.getId() <= 103
E.代碼進化:Lambda 表達式
package com.os.test;
import com.os.model.Employee;
import com.os.service.ObjectDataPredicate;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Demo04 {
private static List<Employee> emps = Arrays.asList(
new Employee(101, "悟空", 18, 9999.99),
new Employee(102, "八戒", 59, 6666.66),
new Employee(103, "唐僧", 28, 3333.33),
new Employee(104, "沙僧", 8, 7777.77),
new Employee(105, "白龍馬", 38, 5555.55)
);
public static void main(String[] args) {
/*
List<Employee> list = filterEmployee(emps, new ObjectDataPredicate<Employee>() {
@Override
public boolean test(Employee employee) {
return employee.getId() <= 103;
}
});//接口回調
*/
List<Employee> list = filterEmployee(emps, (e) -> e.getAge() <= 35);
for (Employee employee : list) {
System.out.println(employee);
}
System.out.println("------------------------------------------");
List<Employee> list2 = filterEmployee(emps, (e) -> e.getSalary() >= 5000);
for (Employee employee : list2) {
System.out.println(employee);
}
}
public static List<Employee> filterEmployee(List<Employee> emps, ObjectDataPredicate<Employee> objectDataPredicate){
List<Employee> list = new ArrayList<>();
for (Employee employee : emps) {
if(objectDataPredicate.test(employee)){
list.add(employee);
}
}
return list;
}
}
Lambda 是一個匿名函數,我們可以把Lambda 表達式理解爲是一段可以傳遞的代碼(將代碼像數據一樣進行傳遞)。可以寫出更簡潔、更靈活的代碼。作爲一種更緊湊的代碼風格,使Java的語言表達能力得到了提升。
二、Lambda基礎語法
Lambda 表達式在Java 語言中引入了一個新的語法元素和操作符。這個操作符爲->
,該操作符被稱爲Lambda 操作符或剪頭操作符。它將Lambda 分爲兩個部分:
- 左側:指定了Lambda 表達式需要的所有參數(對應接口中形參)
- 右側:指定了Lambda 體,即Lambda 表達式要執行的功能。(方法體,可以推斷返回值類型)
A.格式1:無參數,無返回值
package com.os.print.service;
@FunctionalInterface//定義接口函數
public interface Printer01 {
void print();
}
package com.os.print.service;
public class Demo01 {
public static void main(String[] args) {
//之前我們可以使用你們實現類
Printer01 out = new Printer01() {
@Override
public void print() {
System.out.println("匿名實現類");
System.out.println("====>"+Math.random());
}
};
out.print();
//使用Lambda表達式
out = ()-> System.out.println("方法體只有一行,可以省略大括號");
out.print();
out = ()->{
System.out.println("方法體有很多,需要使用大括號搞定");
System.out.println("====>"+Math.random());
};
out.print();
}
}
B.格式2:有一個參數,無返回值
package com.os.print.service;
@FunctionalInterface//定義接口函數
public interface Printer02<T> {
void print(T t);
}
public class Demo02 {
public static void main(String[] args) {
//通過泛型推斷參數e的類型
Printer02<Employee> out01 = (e)-> System.out.println(e);
out01.print(new Employee(999,"悟空",19,25000));
Printer02<Integer> out2 = (e)-> System.out.println(e);
out2.print(999);
Printer02<String> out3 = (e)-> System.out.println(e);
out3.print("西遊記");
}
}
C.格式3:若只有一個參數,小括號可以省略不寫
package com.os.print.service;
import com.os.model.Employee;
public class Demo02 {
public static void main(String[] args) {
//通過泛型推斷參數e的類型
Printer02<Employee> out01 = e-> System.out.println(e);
out01.print(new Employee(999,"悟空",19,25000));
Printer02<Integer> out2 = e-> System.out.println(e);
out2.print(999);
Printer02<String> out3 = e-> System.out.println(e);
out3.print("西遊記");
}
}
D.格式4:有兩個以上的參數,有返回值,並且 Lambda 體中有多條語句
使用系統有的函數接口測試如下:
package com.os.print.service;
import com.os.model.Employee;
import java.util.Comparator;
public class Demo03 {
public static void main(String[] args) {
/*
public interface Comparator<T> {
}
* */
Comparator<Integer> comparator = (x,y)->{
System.out.println("接口函數方法");
return Integer.compare(x,y);
};
}
}
自定義函數接口方法:
package com.os.print.service;
@FunctionalInterface//定義接口函數
public interface Printer03<T> {
T print(T t1,T t2);
}
package com.os.print.service;
import com.os.model.Employee;
import java.util.Comparator;
public class Demo03 {
public static void main(String[] args) {
Printer03<String> out01 = (s1,s2)->{
String str = s1.concat(s2);
return str.toUpperCase();
};
System.out.println(out01.print("abc","efg"));
}
}
自定義函數接口方法兩個參數:
package com.os.print.service;
@FunctionalInterface//定義接口函數
public interface Printer04<T,R> {
R print(T t1, R t2);
}
package com.os.print.service;
import com.os.model.Employee;
public class Demo04 {
public static void main(String[] args) {
Printer04<String, Employee> out = (name,e)->{
e.setName(name);
return e;
};
Employee employee = out.print("西遊記",new Employee());
System.out.println(employee);
}
}
E.格式5:若 Lambda 體中只有一條語句, return 和 大括號都可以省略不寫
package com.os.print.service;
import com.os.model.Employee;
import java.util.Comparator;
public class Demo04 {
public static void main(String[] args) {
Comparator<Integer> comparator = (x, y)->Integer.compare(x,y);
System.out.println(comparator.compare(1,2));
Printer04<String, Employee> out = (name,e)->e;
Employee employee = out.print("西遊記",new Employee());
System.out.println(employee);
}
}
F.格式5:Lambda 表達式的參數列表的數據類型可以省略不寫,因爲JVM編譯器通過上下文推斷出,數據類型,即“類型推斷”
(Integer x, Integer y) -> Integer.compare(x, y); //一般不會使用這種寫法
上述Lambda 表達式中的參數類型都是由編譯器推斷得出的。Lambda 表達式中無需指定類型,程序依然可以編譯,這是因爲javac根據程序的上下文,在後臺推斷出了參數的類型。Lambda 表達式的類型依賴於上下文環境,是由編譯器推斷出來的。這就是所謂的“類型推斷”
lambda語法的總結如下:
- 上聯:左右遇一括號省
- 下聯:左側推斷類型省
- 橫批:能省則省
三、函數式接口
-
只包含一個抽象方法的接口,稱爲函數式接口。
-
你可以通過Lambda 表達式來創建該接口的對象。
- (若Lambda 表達式拋出一個受檢異常,那麼該異常需要在目標接口的抽象方法上進行聲明)。
-
在任意函數式接口上設置
@FunctionalInterface
註解,這樣做可以檢查它是否是一個函數式接口,同時javadoc也會包含一條聲明,說明這個接口是一個函數式接口。
上述的示例中,我們已經定義過函數式接口,但是我們不可能每次都要自己定義函數式接口,太麻煩!所以,Java內置了函數式接口在java.util.function
包下
A.Predicate<T> 斷言型接口
@FunctionalInterface
public interface Predicate<T> {
boolean test(T t);
}
package com.os.print.service;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
public class Demo05 {
public static void main(String[] args) {
List<String> list = Arrays.asList("Hello", "pangsir", "Lambda", "www", "ok");
List<String> strList = filterStr(list, (s) -> s.length() > 3);
for (String str : strList) {
System.out.println(str);
}
}
//需求:將滿足條件的字符串,放入集合中
public static List<String> filterStr(List<String> list, Predicate<String> pre){
List<String> strList = new ArrayList<>();
for (String str : list) {
if(pre.test(str)){
strList.add(str);
}
}
return strList;
}
}
B.Function<T,R> 函數型接口
@FunctionalInterface
public interface Function<T, R> {
R apply(T t);
}
package com.os.print.service;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.function.Predicate;
public class Demo06 {
public static void main(String[] args) {
String newStr = strHandler("\t\t\t 西遊記齊天大聖孫悟空 ", (str) -> str.trim());
System.out.println(newStr);
String subStr = strHandler("西遊記齊天大聖孫悟空", (str) -> str.substring(2, 5));
System.out.println(subStr);
}
//需求:用於處理字符串
public static String strHandler(String str, Function<String, String> fun){
return fun.apply(str);
}
}
C.Supplier<T> 供給型接口
@FunctionalInterface
public interface Supplier<T> {
T get();
}
package com.os.print.service;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Supplier;
public class Demo07 {
public static void main(String[] args) {
List<Integer> numList = getNumList(10, () -> (int)(Math.random() * 100));
for (Integer num : numList) {
System.out.println(num);
}
}
//需求:產生指定個數的整數,並放入集合中
public static List<Integer> getNumList(int num, Supplier<Integer> sup){
List<Integer> list = new ArrayList<>();
for (int i = 0; i < num; i++) {
Integer n = sup.get();
list.add(n);
}
return list;
}
}
D.Consumer<T> 消費型接口
@FunctionalInterface
public interface Consumer<T> {
void accept(T t);
}
package com.os.print.service;
import java.util.function.Consumer;
public class Demo08 {
public static void main(String[] args) {
happy(10000, (m) -> System.out.println("購物消費:" + m + "元"));
}
public static void happy(double money, Consumer<Double> con){
con.accept(money);
}
}
java.util.function包下有很多有用的函數式接口
函數式接口 | 參數類型 | 返回類型 | 用途 |
---|---|---|---|
Consumer<T> 消費型接口 |
T | void | 對類型爲T的對象應用操作,包含方法: void accept(T t) |
Supplier<T> 供給型接口 |
無 | T | 返回類型爲T的對象,包含方法:T get(); |
Function<T, R> 函數型接口 |
T | R | 對類型爲T的對象應用操作。 結果R類型的對象。方法:R apply(T t); |
Predicate<T> 斷定型接口 |
T | boolean | 確定類型爲T的對象是否滿足某約束, boolean 值。含方法boolean test(T t); |
BiFunction<T,U,R> |
T,U | R | 對類型爲T,U參數應用操作,返回R類型的結果。 包含方法爲:R apply(T t,U u); |
UnaryOperator<T> |
T | T | 對類型爲T的對象進行一元運算,並返回T類型的結果。 包含方法爲T apply(Tt); |
BinaryOperator<T> |
T,T | T | 對類型爲T的對象進行二元運算,並返回T類型的結果。 包含方法爲T apply(Tt1,Tt2); |
BiConsumer<T,U> |
T,U | void | 對類型爲T,U參數應用操作。 包含方法爲void accept(T t,U u) |
ToIntFunction<T> ToLongFunction<T> ToDoubleFunction<T> |
T | int long double |
分別計算int、long、double、值的函數 |
IntFunction<R> LongFunction<R> DoubleFunction<R> |
int long double |
R | 參數分別爲int、long、double類型的函數 |
四、方法引用
當要傳遞給Lambda體的操作,已經有實現的方法了,可以使用方法引用!
方法引用:使用操作符“::” 將方法名和對象或類的名字分隔開來。
- 對象::實例方法
- 類::靜態方法
- 類::實例方法
注意:
① <strong style='color:red;'> 方法引用所引用的方法的參數列表與返回值類型,需要與函數式接口中抽象方法的參數列表和返回值類型保持一致!</strong>
② 若Lambda 的參數列表的第一個參數,是實例方法的調用者,第二個參數(或無參)是實例方法的參數時,格式: ClassName::MethodName
A.對象的引用 :: 實例方法名
package com.os.print.service;
import com.os.model.Employee;
import java.util.function.Supplier;
public class Demo09 {
public static void main(String[] args) {
Employee emp = new Employee(101, "張三", 18, 9999.99);
Supplier<String> sup = () -> emp.getName();
System.out.println(sup.get());
System.out.println("----------------------------------");
Supplier<String> sup2 = emp::getName;
System.out.println(sup2.get());
}
}
B.類 :: 靜態方法名
package com.os.print.service;
import java.util.function.BiFunction;
import java.util.function.Supplier;
public class Demo10 {
public static void main(String[] args) {
BiFunction<Double, Double, Double> fun = (x, y) -> Math.max(x, y);
System.out.println(fun.apply(1.5, 22.2));
System.out.println("--------------------------------------------------");
BiFunction<Double, Double, Double> fun2 = Math::max;
System.out.println(fun2.apply(1.2, 1.5));
}
}
C.類 :: 實例方法名
package com.os.print.service;
import com.os.model.Employee;
import java.util.function.BiFunction;
import java.util.function.BiPredicate;
import java.util.function.Function;
public class Demo11 {
public static void main(String[] args) {
BiPredicate<String, String> bp = (x, y) -> x.equals(y);
System.out.println(bp.test("abcde", "abcde"));
System.out.println("-----------------------------------------");
BiPredicate<String, String> bp2 = String::equals;
System.out.println(bp2.test("abc", "abc"));
System.out.println("-----------------------------------------");
Function<Employee, String> fun = (e) -> e.getName();
System.out.println(fun.apply(new Employee()));
System.out.println("-----------------------------------------");
Function<Employee, String> fun2 = Employee::getName;
System.out.println(fun2.apply(new Employee()));
}
}
若Lambda 的參數列表的第一個參數,是實例方法的調用者,第二個參數(或無參)是實例方法的參數時,格式: ClassName::MethodName
D.構造方法引用 ClassName::new
格式:ClassName::new
與函數式接口相結合,自動與函數式接口中方法兼容。
可以把構造器引用賦值給定義的方法,與構造器參數列表要與接口中抽象方法的參數列表一致!
package com.os.print.service;
import com.os.model.Employee;
import java.util.function.BiFunction;
import java.util.function.BiPredicate;
import java.util.function.Function;
public class Demo12 {
public static void main(String[] args) {
Function<String, Employee> fun = Employee::new;
System.out.println(fun.apply("悟空"));
BiFunction<String, Integer, Employee> fun2 = Employee::new;
System.out.println(fun2.apply("八戒",18));
}
}
E.數組引用
package com.os.print.service;
import com.os.model.Employee;
import java.util.function.BiFunction;
import java.util.function.Function;
public class Demo13 {
public static void main(String[] args) {
Function<Integer, String[]> fun = (e) -> new String[e];
String[] strs = fun.apply(10);
System.out.println(strs.length);
System.out.println("--------------------------");
Function<Integer, Employee[]> fun2 = Employee[] :: new;
Employee[] emps = fun2.apply(20);
System.out.println(emps.length);
}
}