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javac app.java

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// single line comment

/* 
    multi-line comment
*/

/** 
*   doc comment 
*/

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public static void main(String [] args);

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class Guru99 {
    static int a = 1; //static variable  
    int data = 99; //instance variable  
    void method() {
        int b = 90; //local variable  
    }
}

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if (con1) {
	// do somethin
} else if (con2) {
	// do something
} else {
	// do something
}

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switch(expr) {
	case val1:
		// do something
		break; 
	case val2:
		// do something
		break;
	default:
		// do something
		break;
}

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// 1
for(int i = 0; i <= n; i++) {
	// do something
}

// 2 for each
int arr[] = {1, 2, 3};
for(int i:arr) {
	// do something
}

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while (condition) {
	// do something
}

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do {
	// something 
} while ();

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import static java.lang.System.out;
import static java.lang.Math.*;
...
out.println("1");
sqrt(2);

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try {
	int a = 5;
	int b = 0;
	int c = a/b;
} catch (ArithmeticException e) {
	System.out.println(e.getMessage());  // Out: by zero
} finally {
	// do something
    // 即使 try/catch block 有 return 语句, finally 也会执行
}

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returnType method_name(paramList) throws Exception 1,Exception2, ... { ... }

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throw new IllegalArgumentException("用户名长度必须大于 8 位！");

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public class Test {
	public void test_throws() throws IOException {
		FileInputStream fis = new FileInputStream("a.txt");
	}

	public boolean test_throw(String username) {
		boolean res = false;
		if (username.length() < 8) {
			throw new IllegalArgumentException("用户名长度不足8位.")
		} else {
			res = true;
		}
		return res;
	}

	public static void main(String[] args) {
		// throws
		try {
			test_throws();
		} catch (IOException e) {
			System.out.println(e);  //
		}

		// throw
		Scanner scanner = new Scanner(System.in);
		System.out.println("请输入用户名：");
		String username = input.next();
		try {
			boolean con = test_throw(username);
		} catch (IllegalArgumentException e) {
			System.out.println(e);  // Out: java.lang.IllegalArgumentException: 用户名长度不足8位.
		}
	}
}

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// 两种方式

// 处理
void foo() {
    try {
        laundry.doLaundry();
    } catch (ClothingException cex) {
        // do something
    }
}

// duck
// 于是乎调用 foo() 的时候就要 处理 or duck
void foo() throw ClothingException {
    laundry.doLaundry();
}

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import java.util.vector;
import java.util.*; // 引入所有类, 但子包的不会

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// Note static keyword after import.
import static java.lang.System.*;
class StaticImportDemo
{
   public static void main(String args[])
   {      
        // We don't need to use 'System.out' 
        // as imported using static.
        out.println("GeeksforGeeks");
   }

}

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// 两者都包含 Date 类
import java.util.*;
import java.sql.*;

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> SET CLASSPATH=.;c:\javaproject\classes;d:\tomcat\lib\servlet-api.jar

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> java –classpath c:\javaproject\classes com.abc.project1.subproject2.MyClass3

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import package_one.ClassTwo;
import package_name.ClassOne;
public class Testing {
    public static void main(String[] args){
        ClassTwo a = new ClassTwo();
        ClassOne b = new ClassOne();
        a.methodClassTwo();
        b.methodClassOne();
    }
}

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// Java < 11
Path path = Paths.get("c:/dev/readme.txt");

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// Java 11+
Path path = Path.of("c:/dev/readme.txt");
path = Path.of("c:", "dev", "readme.txt");
path = Path.of("c:", "dev").resolve("readme.txt");
path = Path.of(new URI("file:///c:/dev/readme.txt");

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boolean file_exists = Files.exists(path);

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FileTime lastModifiedTime = Files.getLastModifiedTime(path);
UserPrincipal owner = Files.getOwner(path);

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// 在Temp文件夹创建带固定前缀的临时文件
Path tempFile1 = Files.createTempFile("prefixOrNull", ".txt"); 

// 在指定路径创建临时文件
Path tempFile2 = Files.createTempFile(path.getParent(), "prefix", ".jpg"); 

// 在Temp文件夹创建临时目录
Path tempDir = Files.createTempDirectory("prefix");

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Path newDir = Files.createDirectory(path.getParent().resolve("newDir"));
Path tempFile1 = Files.createFile(newDir.resolve("newFile.txt")); 

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Path path = Path.of("c:\\dev\\licenses\\windows\\readme.txt");
try {
    Set<PosixFilePermission> permissions = Files.getPosixFilePermissions(path);
    System.out.println("permissions = " + permissions);
} catch (UnsupportedOperationException e) {
    System.err.println("Looks like you're not running on a posix file system");
}

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Path utf8File = Files.createTempFile("utf8", ".txt");
Files.writeString(utf8File, "A String üü");

Path iso88591 = File.createTempFile("iso88591", ".txt");
Files.writeString(iso88591, "A String üü", StandardCharsets.ISO_8859_1);

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Files.write(anotherIso88591File, "this is my string ää öö üü".getBytes(StandardCharsets.ISO_8859_1));

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Files.write(filename, "string".getBytes(), StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING, StandardOpenOption.WRITE);

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try (BufferedWriter bufferedWriter = Files.newBufferedWriter(utfFile)) {
    // handle reader
}

try (OutputStream os = Files.newOutputStream(utfFile)) {
    // handle outputstream
}

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String s = Files.readString(utfFile); // utf8
s = Files.readString(utfFile, StandardCharsets.ISO_8859_1);

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String s = new String(Files.readAllBytes(utfFile), StandardCharsets.UTF_8);

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try (BufferedReader bufferedReader = Files.newBufferedReader(utfFile)) {
	// handle reader
}
try (InputStream is = Files.newInputStream(utfFiles)) {
	// handle inputstream
}

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// 移动 test.jpg → c:\temp\test.jpg
Files.move(filepath, Path.of(target_path_string).resolve(filepath.getFileName().toString())[,options]); 
// StandardCopyOption.REPLACE_EXISTING
// catch (FileAlreadyExistsException e)
  
// 删除 空dir
// 删除 非空文件夹, 思路是用 Files.walk(dir) 遍历
try {
	Files.delete(path);
} catch (DirectoryNotEmptyException e) {
	e.printStackTrace();
}

// list files
// 同级同目录下
try (var files = Files.list(temDir)) {
	files.forEach(System.out::println);
}
// 指定文件类型
try (var files = Files.newDirectoryStream(tmpDir, ".txt")) {
	files.forEach(System.out::println);
}
// -r
try (var files = Files.walk(tmpDir)) {
	files.forEach(System.out::println);
}