為什麼要寫這篇文章
經過了若干年的發展，Java逐步從java8升級為java11，java17。
讓我們對比學習一下最新一版的LTS版本和java8比起來讓程式碼簡化了多少。
- 文字塊（Text Blocks）。
這個寫法有些類似於 javascript、 Lua等指令碼語言。方便識別html、json等格式複雜的字串。
public class StringTest {
public static void main(String[] args) throws Exception {
// 傳統寫法
String json =
"{\n" +
" \"key\":\"a\",\n" +
" \"value\":\"b\"\n" +
"}";
// 優化後寫法
String json2 = """
{
"key":"a",
"value":"b"
}
""";
// 返回為 true
System.out.println(json == json2);
}
}
- 本地變數型別推斷（Local Variable Type Inference）
這一點也是在一些指令碼語言中常見的，類似於 var 表示變數，val 表示常量。
public static void main(String[] args) throws Exception {
//集合
// immutable map build
var map = Map.of(
"cat", "貓",
"dog", "狗",
"fish", "魚");
// immutable set build
var set = Set.of("1", "2", "3");
// immutable list build
var list = List.of(1, 2, 3, 4, 5);
// 迴圈語句
for (var i = 1; i < list.size(); i++) {
System.out.println(i);
}
for (var i : list) {
System.out.println(i);
}
// 異常
try (var in = new ByteArrayInputStream("123".getBytes())) {
System.out.println(new String(in.readAllBytes(), "utf-8"));
} catch (Exception e) {
System.out.println(e);
}
// lambda 表示式 意思相同
BiFunction<Integer, Integer, Integer> biFunction = (a, b) -> a + b;
BiFunction<Integer, Integer, Integer> biFunction2 = (var a, var b) -> a + b;
}
- switch
public static void main(String[] args) throws Exception {
var eating = Eating.BREAKFAST;
String eatingZnString = "";
// 傳統寫法
switch (eating) {
case BREAKFAST:
case LUNCH:
eatingZnString = "早午飯";
break;
case DINNER:
eatingZnString = "晚飯";
break;
default:
throw new Exception();
}
System.out.println(eatingZnString);
// 優化後寫法
System.out.println(
switch (eating) {
case BREAKFAST,LUNCH -> "早午飯";
case DINNER -> "晚飯";
default -> throw new Exception();
}
);
}
instance of操作符的模式匹配(Pattern Matching for the instanceof Operator)
interface Animal {}
class Cat implements Animal {
public void mew() {
System.out.println("喵");
}
}
class Dog implements Animal {
public void woof() {
System.out.println("汪");
}
}
public class Test {
// 傳統寫法
public static void sounds(Animal animal) {
if (animal instanceof Cat) {
Cat cat = (Cat) animal;
cat.mew();
} else if (animal instanceof Dog) {
Dog dog = (Dog) animal;
dog.woof();
} else {
throw new IllegalArgumentException("沒有這種動物的叫聲");
}
}
// 優化寫法
public static void betterSounds(Animal animal) {
if (animal instanceof Cat cat) {
cat.mew();
} else if (animal instanceof Dog dog) {
dog.woof();
} else {
throw new IllegalArgumentException("沒有這種動物的叫聲");
}
}
}
- record 類
// 傳統類
public record People(String name, int age) {
public People(String name, int age) {
this.name = name;
this.age = age;
}
public String name() {
return this.name;
}
public int age() {
return this.age;
}
public boolean equals(People people) {...}
public int hashCode() {...}
public String toString() {...}
}
// 優化後的類
public record People (String name, int age){
}
// 更多用法
public record People (String name, int age){
// 靜態欄位
static int teenageAge;
// 靜態初始化
static {
teenageAge = 17;
}
// 靜態方法
public static People buildTeenage(String name) {
return new People(name , teenageAge);
}
// 優化後的構造方法
public People {
if (age < 0) {
throw new IllegalArgumentException("年齡不能小於0");
}
}
}