文件写入- PrintStream附加

Question

我试图将一些信息附加到一个文本文件中，但该文件只显示了写入的最后一个元素。

有许多Engineer，但它只将读取的最后一个元素打印到文件中。

例如：

Engineer e = new Engineer(firstName,surName,weeklySal);
PrintStream writetoEngineer = new PrintStream(new File ("Engineer.txt"));

//This is not append. Only print. Overwrites the file on each item.
writetoEngineer.append(e.toString() + " "  + e.calculateMontly(weeklySal));

Answer 1

我不知道您要在哪里关闭文件。我也没看到你在读什么。

我假设您想要追加到文件中，而不是每次都覆盖它。在这种情况下，您需要使用FileOutputStream的append选项，因为这不是默认行为。

PrintStream writetoEngineer = new PrintStream(
     new FileOutputStream("Engineer.txt", true)); 

顺便说一句：e.toString() + " "与e + " "几乎相同，只是如果e为空，它不会抛出异常。

Answer 2

由于给定代码片段的代码不是一个自包含的可编译示例(尽管它很简单)，因此我可以猜测PrintStream是在循环中创建的，每次迭代都会遍历Engineer集合。这将导致文件被截断，如PrintStream的构造函数javadoc所示：

参数：

file -用作此打印流目标的文件。如果该文件存在，则它将被截断为零大小；否则，将创建一个新文件。输出将写入文件并进行缓冲。

尝试以下示例代码：

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintStream;
import java.util.ArrayList;
import java.util.List;


public class PrintEngineers {

    public static class Engineer {
    
        private final String firstName;
        private final String surName;
        private final int weeklySal;
    
        public Engineer(String firstName, String surName, int weeklySal) {
            super();
            this.firstName = firstName;
            this.surName = surName;
            this.weeklySal = weeklySal;
        }

        public int calculateMonthly() {
            return weeklySal * 4; // approximately
        }
    
        @Override
        public String toString() {
            return firstName + " " + surName;
        }
    }

    /**
     * @param args
     * @throws FileNotFoundException 
     */
    public static void main(String[] args) throws FileNotFoundException {
    
        Engineer e1 = new Engineer("first1", "sur1", 100);
        Engineer e2 = new Engineer("first2", "sur2", 200);
        Engineer e3 = new Engineer("first3", "sur3", 300);

        List<Engineer> engineers = new ArrayList<>(3);
        engineers.add(e1);
        engineers.add(e2);
        engineers.add(e3);

        // instanciate PrintStream here, before the loop starts
        PrintStream writetoEngineer = new PrintStream(new File("Engineer.txt"));
        for (Engineer engineer : engineers) {
            // new PrintStream(...) here truncates the file (see javadoc)               //This is not append.Only print.Refresh file on each item 
            writetoEngineer.append(engineer.toString()).append(' ')
                        .append("" + engineer.calculateMonthly()).append('\n'); 
        
        }
    }

}