如何使用 Java PrintWriter 将 void 类型方法的结果打印到文件中

Question

我正在为我的 Java 编程课程做练习，问题是这样的：

编写一个名为 printTree 的方法，该方法根据传递给过程的高度值将星号三角形输出到文件中。在 main 方法中测试这个方法。

• 例如高度为 3 的三角形应输出到名为 file14 的文件：

我只是不确定如何将 void 返回写入我在 main 方法中创建的文件。我想尽量减少除 FileWriter 之外的任何其他 java.io 方法的导入，但感谢您提供任何帮助。

import java.io.PrintWriter;

public class OutputToFile14 {

    public static void main(String[] args) throws Exception{
        
        //Creating PrintWriter
        PrintWriter output = new PrintWriter("file14.txt");
        
        //writing method output to file
        output.write(printTree(4));
        
        //saving file
        output.close();
    }

    public static void printTree (int height) throws IOException{
        
        for (int i = 0; i < height; i++) {
            for (int j = 0; j < height; j++) {
                if (j < i) {
                    System.out.print("*");
                }
            }
            System.out.println();
        }
    } 
}

Answer 1

四个观察。 System.out 是 PrintStream （您可以将 PrintStream 传递给您的方法）。 try-with-Resources 允许您消除显式的 close() 调用。使用System.getProperty("user.home") 允许您直接写入主文件夹（这很方便）。并在您的内部循环中使用j < i 而不是if (j < i)。喜欢，

public static void main(String[] args) throws Exception {
    try (PrintStream output = new PrintStream(
            new File(System.getProperty("user.home"), "file14.txt"))) {
        printTree(output, 4);
    }
}

public static void printTree(PrintStream out, int height) throws IOException {
    for (int i = 0; i < height; i++) {
        for (int j = 0; j < i; j++) {
            out.print("*");
        }
        out.println();
    }
}

此外，从 Java 11 开始，

public static void printTree(PrintStream out, int height) throws IOException {
    for (int i = 0; i < height; i++) {
        out.println("*".repeat(i)); // You might want "*".repeat(1 + i)
    }
}

Answer 2

你可以这样解决

import java.io.PrintWriter;

public class OutputToFile14 {

    public static void main(String[] args) throws Exception{
        
        //Creating PrintWriter
        PrintWriter output = new PrintWriter("file14.txt");
        
        //writing method output to file
        output.write(printTree(4));
        
        //saving file
        output.close();
    }

    public static String printTree (int height) throws IOException{
        String output = "";
        
        for (int i = 0; i < height; i++) {
            for (int j = 0; j < height; j++) {
                if (j < i) {
                    System.out.print("*");
                    output += "*";
                }
            }
            System.out.println();
            output += "\r\n";
        }
        return output;
    } 
}

这是一种有点难看的快速解决方法。

import java.io.PrintWriter;

public class OutputToFile14 {

    public static void main(String[] args) throws Exception{
        
        //Creating PrintWriter
        PrintWriter output = new PrintWriter("file14.txt");
        
        //writing method output to file
        //output.write(printTree(4));
        printTree(4, output);
        
        //saving file
        output.close();
    }

    public static void printTree (int height, PrintWriter pw) throws IOException{
        
        for (int i = 0; i < height; i++) {
            for (int j = 0; j < height; j++) {
                if (j < i) {
                    System.out.print("*");
                    pw.write("*");
                }
            }
            System.out.println();
            pw.write("\r\n");
        }
    } 
}