1.java11 witch表达式
1.传统switch的弊端
传统的switch声明语句(switch statement)在使用中有一些问题：
匹配是自上而下的，如果忘记写break, 后面的case语句不论匹配与否都会执行；
所有的case语句共用一个块范围，在不同的case语句定义的变量名不能重复；
不能在一个case里写多个执行结果一致的条件；
整个switch不能作为表达式返回值；
package com.atguigu.java;
import org.junit.Test;
/**
* @author shkstart
* @create 2019 下午 3:13
*/
public class SwitchTest1 {
//java 12的新特性
@Test
public void testSwitch1(){
Fruit fruit = Fruit.APPLE;
switch(fruit) {
case PEAR -> System.out.println("4");
case APPLE,GRAPE,MANGO -> System.out.println("5");
case ORANGE,PAPAYA -> System.out.println("6");
default -> throw new IllegalStateException("No Such Fruit:" + fruit);
}
}
@Test
public void testSwitch2(){
int numberOfLetters;
Fruit fruit = Fruit.APPLE;
numberOfLetters = switch(fruit) {
case PEAR -> 4;
case APPLE,GRAPE,MANGO -> 5;
case ORANGE,PAPAYA -> 6;
default -> throw new IllegalStateException("No Such Fruit:" + fruit);
};
System.out.println(numberOfLetters);
}
}
2.java11 JVM 常量 API:
package com.atguigu.java;
import java.util.Optional;
/**
* @author shkstart
* @create 2019 下午 5:18
*/
public class StringConstantTest {
private static void testDescribeConstable() {
System.out.println("======test java 12 describeConstable======");
String name = "尚硅谷Java高级工程师";
Optional<String> optional = name.describeConstable();
System.out.println(optional.get());
}
public static void main(String[] args) {
testDescribeConstable();
}
}
3.java11 支持压缩数字格式化:
@Test public void testCompactNumberFormat(){ var cnf = NumberFormat.getCompactNumberInstance(Locale.CHINA, NumberFormat.Style.SHORT); System.out.println(cnf.format(1_0000)); System.out.println(cnf.format(1_9200)); System.out.println(cnf.format(1_000_000)); System.out.println(cnf.format(1L << 30)); System.out.println(cnf.format(1L << 40)); System.out.println(cnf.format(1L << 50)); }
输出:
1万
2万
100万
11亿
1兆
1126兆
4.java11 String新增方法:
package com.atguigu.java;
import org.junit.Test;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;
/**
* @author shkstart
* @create 2019 下午 8:19
*/
public class StringTest {
@Test
public void testTransform(){
String info1 = " hello".transform(info -> info + "world ");
System.out.println(info1);
}
// hello --> helloworld --> HELLOWORLD --> HELLOWORLD
//映射：java 8 中 Stream API :map()
educe()
@Test
public void testTransform1(){
var info1 = "hello".transform(info -> info + "world").transform(String::toUpperCase).transform(String::trim);
System.out.println(info1);
}
@Test
public void testTransform2(){
System.out.println("======test java 12 transform======");
List<String> list1 = List.of("Java", " Python", " C++ ");
List<String> list2 = new ArrayList<>();
list1.forEach(element -> list2.add(element.transform(String::strip)
.transform(String::toUpperCase)
.transform(e -> "Hi," + e))
);
list2.forEach(System.out::println);
}
@Test
public void testTransform3(){
System.out.println("======test java 12 transform======");
List<String> list1 = List.of("Java", " Python", " C++ ");
Stream<String> strStream = list1.stream().map(word -> word.strip()).map(String::toUpperCase).map(word -> "hello," + word);
List<String> list2 = strStream.collect(Collectors.toList());
list2.forEach(System.out::println);
}
//String中的indent()
@Test
public void testIndent() {
System.out.println("======test java 12 indent======");
String result = "Java
Python
C++".indent(3); //如果没有换行在行首补上3个空格,如果存在换行在每行的行首补上3个空格
System.out.println(result);
}
}
5.java11 Files新增mismatch方法:
//比较两个文件的对应行,如果所有行都相同返回0, 否则返回行编号默认从0开始
package com.atguigu.java;
import org.junit.Test;
import java.io.FileWriter;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
/**
* @author shkstart
* @create 2019 下午 8:46
*/
public class FilesTest {
//使用IDEA的单元测试方法，默认的相对路径是在当前module下
//IO : File
//NIO 2 : Files操作本地文件的工具类 ； Path:替换原有的File ; Paths :实例化Path
@Test
public void testFilesMismatch() throws IOException {
FileWriter fileWriter = new FileWriter("tmp\a.txt");
fileWriter.write("a");
fileWriter.write("b");
fileWriter.write("c");
fileWriter.close();
FileWriter fileWriterB = new FileWriter("tmp\b.txt");
fileWriterB.write("a");
fileWriterB.write("b");
fileWriterB.write("c");
fileWriterB.close();
System.out.println(Files.mismatch(Path.of("tmp/a.txt"),Path.of("tmp/b.txt")));
}
}
6.java13 switch表达式（预览):
package com.atguigu.java;
import org.junit.Test;
/**
* @author shkstart
* @create 2019 下午 9:12
*/
public class SwitchTest {
@org.junit.Test
public void testSwitch1(){
String x = "1";
int i;
switch (x) {
case "1":
i=1;
break;
case "2":
i=2;
break;
default:
i = x.length();
break;
}
System.out.println(i);
}
//java 13：switch中引入yield，可以返回值给相应的变量
@Test
public void testSwitch2(){
String x = "1";
int i = switch(x){
case "1" -> 5;
case "2" -> 6;
default -> {
yield 7;
}
};
System.out.println(i);
}
@Test
public void testSwitch3(){
String x = "3";
int i = switch(x){
case "1" : yield 5;
case "2" : yield 6;
default : yield 7;
};
System.out.println(i);
}
//yield 和 return
//yield:结束switch结构
//return:结束方法
}
7. java13 文本块:
package com.atguigu.java;
import org.junit.Test;
import java.util.Objects;
/**
* java 13新特性：TextBlock （预览）
* @author shkstart
* @create 2019 下午 3:00
*/
public class TextBlockTest {
@Test
public void test1(){
String html = "<html>
" +
" <body>
" +
" <p>Hello, 尚硅谷</p>
" +
" </body>
" +
"</html>";
System.out.println(html);
System.out.println();
String html1 = """
<html>
<body>
<p>Hello, 尚硅谷</p>
</body>
</html>
""";
System.out.println(html1);
}
@Test
public void test2(){
String sql = "select employee_id,last_name,salary,department_id
" +
"from employees
" +
"where department_id in (40,50,60)
" +
"order by department_id asc";
System.out.println(sql);
String sql1 = """
select employee_id,last_name,salary,department_id
from employees
where department_id in (40,50,60)
order by department_id asc
""";
System.out.println(sql1);
}
//////////////////////////////////////////////////////////////////////////
//关于TextBlock的基本使用
@Test
public void test3(){
//以开始分隔符的行终止符后的第一个字符开始
//以结束分隔符的第一个双引号之前的最后一个字符结束
String text1 = """
abc""";
String text2 = "abc";
System.out.println(text1 == text2);//text1和text2都指向了字符串常量池中唯一定义的abc字面量
String text3 = """
abc
""";
System.out.println(text1.length());//3
System.out.println(text3.length());//4 test3后面还有一个换行符
}
//空字符串的表示
@Test
public void test4(){
String text1 = "";
System.out.println(text1.length()); //1
String text2 = """
""";
System.out.println(text2.length()); //1
System.out.println(test1 == test2); //true
}
//错误的写法
@Test
public void text5(){
// String a = """"""; // 开始分隔符后没有行终止符
// String b = """ """; // 开始分隔符后没有行终止符
// String c = """
// "; // 没有结束分隔符
//String d = """
// abc def
// """; // 含有未转义的反斜线（请参阅下面的转义处理）
//
// String e = "abc def";
}
//编译器在编译时会删除掉这些多余的空格(结尾会直接去掉,但是前面的空格比较特殊,会和结束分隔符进行相应的比较,如果是和结束分隔符对齐的则去电,去个不是对齐的则不去掉:如下)
@Test
public void test6(){
String text1 = """
abc
""";
System.out.println(text1.length());//4
String text2 = """
abc //这里我加了两个空格
""";
System.out.println(text1.length());//6
String text2 = """
abc //这里我加了两个空格
"""; //这里我也加了两个空格
System.out.println(text1.length());//4
}
// 转义字符
@Test
public void test7(){
String html = """
<html>
<body>
<p>Hello, world</p>
</body>
</html>
""";
System.out.println(html);
}
//在文本块内自由使用"是合法的
@Test
public void test8(){
String story = """
"When I use a word," Humpty Dumpty said,
in rather a scornful tone, "it means just what I
choose it to mean - neither more nor less."
"The question is," said Alice, "whether you
can make words mean so many different things."
"The question is," said Humpty Dumpty,
"which is to be master - that's all."
""";
System.out.println(story);
String code =
"""
String text = """
A text block inside a text block
""";
""";
System.out.println(code);
}
//文本块连接
@Test
public void test9(){
String type = "String";
String code = """
public void print(""" + type + """
o) {
System.out.println(Objects.toString(o));
}
""";
System.out.println(code);
//改进：可读性更好 ---方式1
String code1 = """
public void print($type o) {
System.out.println(Objects.toString(o));
}
""".replace("$type", type);
System.out.println(code1);
//方式2
String code2 = String.format("""
public void print(%s o) {
System.out.println(Objects.toString(o));
}
""", type);
System.out.println(code2);
//方式3
String code3 = """
public void print(%s object) {
System.out.println(Objects.toString(object));
}
""".formatted(type);
System.out.println(code3);
}
}