虽然像FileOutputStream、OutputStreamWriter这些输出流可以输出信息，但是其会要求把数据先处理为字符串或字节数组，使用不便。为此在Java IO体系专门提供了两个打印流——PrintStream 字节打印流、PrintWriter 字符打印流。二者均重载了print、printf 等函数，方便开发者直接输出打印其它类型的数据

PrintStream 字节打印流

PrintStream其支持多种构造方式，如下所示。值得一提的是PrintStream是对字节输出流的包装，所以即使是从文件中直接创建字节打印流，其构造方法内部也是先构造一个FileOutputStream字节输出流

public class PrintStream extends FilterOutputStream {
    ...
    // 从文件直接创建字节打印流
    public PrintStream(File file) throws FileNotFoundException;
    // 从文件直接创建字节打印流
    public PrintStream(File file, String csn) throws FileNotFoundException, UnsupportedEncodingException;
    // 从字节输出流创建字节打印流
    public PrintStream(OutputStream out);
    // 从文件直接创建字节打印流
    public PrintStream(String fileName) throws FileNotFoundException;
    ...
}

这里一个实例来演示如何通过 PrintStream 字节打印流来直接输出数据


// 字节打印流 测试用例
public static void testPrintStream() throws FileNotFoundException, UnsupportedEncodingException {
    String file1 = "E:\\TestCode\\JavaTest\\src\\main\\resources\\PrintStreamTest\\w1.txt";
    String file2 = "E:\\TestCode\\JavaTest\\src\\main\\resources\\PrintStreamTest\\w2.txt";
    String file3 = "E:\\TestCode\\JavaTest\\src\\main\\resources\\PrintStreamTest\\w3.txt";

    // 从字节流中创建字节打印流
    FileOutputStream out = new FileOutputStream(file1);
    PrintStream printStream1 = new PrintStream(out, true, "gbk");
    getList().forEach( e -> {
            String separator = " : ";
            printStream1.print(e.getName());
            printStream1.print(separator);
            printStream1.print(e.getAge());
            printStream1.print(separator);
            printStream1.println(e.getSalary());
    });
    printStream1.flush();
    printStream1.close();

    // 直接从文件中创建字符打印流
    PrintStream printStream2 = new PrintStream(file2,"gbk");
    getList().forEach( e ->{
        String separator = " | ";
        printStream2.print(e.getName());
        printStream2.print(separator);
        printStream2.print(e.getAge());
        printStream2.print(separator);
        printStream2.println(e.getSalary());
    });
    printStream2.flush();
    printStream2.close();

    // 直接从文件中创建字符打印流
    PrintStream printStream3 = new PrintStream(file3, "gbk");
    getList().forEach( e ->     // 格式化输出
        printStream3.printf("Name: %s\tAge: %d\tSalary: %.2f\n",
            e.getName(), e.getAge(), e.getSalary())
    );
    printStream3.flush();
    printStream3.close();
}

// 构造一个 Employee 的列表 作为 数据来源
public static List<Employee> getList() {
    Employee e1 = Employee.builder()
        .name("Aaron")
        .age(23)
        .salary(1000.00)
        .build();

    Employee e2 = Employee.builder()
        .name("Tony")
        .age(13)
        .salary(20.00)
        .build();

    Employee e3 = Employee.builder()
        .name("David")
        .age(36)
        .salary(300.00)
        .build();

    Employee e4 = Employee.builder()
            .name("艾米")
            .age(46)
            .salary(1300.00)
            .build();


    List<Employee> list = new LinkedList<>();
    list.add(e1);
    list.add(e2);
    list.add(e3);
    list.add(e4);
    return list;
}

从下面的测试结果可以看出，我们不仅可以通过 print、println 方法直接输出打印其它类型的数据，还可以通过printf实现格式化输出

PrintWriter 字符打印流

同样地，PrintWriter 也是对字符输出流的包装，所以虽然其构造函数支持从文件、字节输入流中构造，但是构造函数内部均先构造了一个BufferedWriter实例

public class PrintWriter extends Writer {
    ...
    public PrintWriter(File file) throws FileNotFoundException;
    public PrintWriter(File file, String csn)
    public PrintWriter(String fileName) throws FileNotFoundException;
    public PrintWriter(OutputStream out);
    public PrintWriter (Writer out);
    ...
}

这里一个实例来演示如何通过 PrintWriter 字符打印流来直接输出数据

public static void testPrintWriter() throws FileNotFoundException, UnsupportedEncodingException {
    String file1 = "E:\\TestCode\\JavaTest\\src\\main\\resources\\PrintWriterTest\\w1.txt";
    String file2 = "E:\\TestCode\\JavaTest\\src\\main\\resources\\PrintWriterTest\\w2.txt";
    String file3 = "E:\\TestCode\\JavaTest\\src\\main\\resources\\PrintWriterTest\\w3.txt";


    // 直接从文件中创建字符打印流
    PrintWriter printWriter1 = new PrintWriter(file1,"gbk");
    getList().forEach( e -> {
        String separator = " - ";
        printWriter1.print(e.getName());
        printWriter1.print(separator);
        printWriter1.print(e.getAge());
        printWriter1.print(separator);
        printWriter1.println(e.getSalary());
    });
    printWriter1.flush();
    printWriter1.close();

    // 从字节流中创建字符打印流
    FileOutputStream fileOutputStream2 = new FileOutputStream(file2);
    PrintWriter printWriter2 = new PrintWriter(fileOutputStream2);
    getList().forEach( e -> {
        String separator = " % ";
        printWriter2.print(e.getName());
        printWriter2.print(separator);
        printWriter2.print(e.getAge());
        printWriter2.print(separator);
        printWriter2.println(e.getSalary());
    });
    printWriter2.flush();
    printWriter2.close();

    // 从字符流中创建字符打印流
    FileOutputStream fileOutputStream3 = new FileOutputStream(file3);
    OutputStreamWriter outputStreamWriter = new OutputStreamWriter(fileOutputStream3, "gbk");
    BufferedWriter bufferedWriter = new BufferedWriter(outputStreamWriter);
    PrintWriter printWriter3 = new PrintWriter(bufferedWriter);
    getList().forEach( e ->
            printWriter3.printf("Name: %s\tAge: %d\tSalary: %.2f\n",
                    e.getName(), e.getAge(), e.getSalary())
    );
    printWriter3.flush();
    printWriter3.close();
}

可以看出，PrintWriter 与 PrintStream 相比，在输出打印的使用上，并无明显区别

System.out.println

我们在日常开发经常会用 System.out.println 来向控制台打印输出信息，其实 out 就是System类中一个PrintStream类型的静态成员变量

参考文献

Java核心技术·卷II 凯.S.霍斯特曼著

xyZGHio

Java IO：PrintStream、PrintWriter 打印流

PrintStream 字节打印流

PrintWriter 字符打印流

System.out.println

参考文献