Problem statement
In a compiled .jar file that I execute on wsl Ubuntu, I run command: task reportToGetUrgencyOfAllTasks which returns a list of tasks with columns: id, uuid, urgency. I can inspect the list, it is printed to terminal, and it consist of 1793 tasks and 1798 lines (a header and some irrelevant messages about requiring sync.). When I count the number of objects with reader.lines().count() it returns 1798 and consumes the lines as expected.
If I create a while loop with:
long counter = 0;
while (reader.lines().iterator().hasNext()) {
counter++;
System.out.println("counter=" + counter);
}
It lists numbers 1 to 1798, which is weird because I thought the .hasNext() did not consume items, but only checked whether the next element existed or not, without taking it from the stream. (As suggested here:https://hajsoftutorial.com/iterator-hasnext-and-next/). I expected an infinite loop of numbers because the iterator would stay at the first element forever after the hasNext().
So then if I want to take all elements from the reader and put them into an ArrayList with:
ArrayList<String> capturedCommandOutput = new ArrayList<String>();
while (reader.lines().iterator().hasNext()) {
counter++;
capturedCommandOutput.add(reader.lines().iterator().next());
}
It skips every other line.
Question
Why does hasNext() consume elements from the iterator in this scenario?
Complete code For an MWE one needs to install taskwarrior on WSL Ubuntu, however, I think this is a Java problem since I can see that the reader does contain all lines/information, hence for completeness: the complete method that runs the command is:
package customSortServer;
import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Map;
import java.util.StringJoiner;
public class RunCommandsLongOutput2 {
/**
* This executes the commands in terminal. Additionally it sets an environment
* variable (not necessary for your particular solution) Additionally it sets a
* working path (not necessary for your particular solution)
*
* @param commandData
* @param ansYes
* @throws Exception
*/
public static ArrayList<String> executeCommands(Command command, Boolean ansYes) {
ArrayList<String> capturedCommandOutput = new ArrayList<String>();
File workingDirectory = new File(command.getWorkingDirectory());
// create a ProcessBuilder to execute the commands in
ProcessBuilder processBuilder = new ProcessBuilder(command.getCommandLines());
// this is set an environment variable for the command (if needed)
if (command.isSetEnvVar()) {
processBuilder = setEnvironmentVariable(processBuilder, command);
}
// this can be used to set the working directory for the command
if (command.isSetWorkingPath()) {
processBuilder.directory(workingDirectory);
}
// execute the actual commands
try {
Process process = processBuilder.start();
System.out.println("Started");
if (command.isGetOutput()) {
// capture the output stream of the command
BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
// System.out.println(reader.lines().count());
// long counter = 0;
while (reader.lines().iterator().hasNext()) {
capturedCommandOutput.add(reader.lines().iterator().next());
}
// while (reader.lines().iterator().hasNext()) {
// counter++;
// System.out.println("line=" + reader.lines().iterator().next());
// }
}
// connect the output of your command to any new input.
// e.g. if you get prompted for `yes`
new Thread(new SyncPipe(process.getErrorStream(), System.err)).start();
new Thread(new SyncPipe(process.getInputStream(), System.out)).start();
PrintWriter stdin = new PrintWriter(process.getOutputStream());
// This is not necessary but can be used to answer yes to being prompted
if (ansYes) {
stdin.println("yes");
}
// write any other commands you want here
stdin.close();
// If the command execution led to an error, returnCode!=0, or not (=0).
int returnCode = process.waitFor();
System.out.println("Return code = " + returnCode);
} catch (IOException e1) {
e1.printStackTrace();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
// return output if required:
return capturedCommandOutput;
}
/**
* source: https://stackoverflow.com/questions/7369664/using-export-in-java
*
* @param processBuilder
* @param varName
* @param varContent
* @return
*/
private static ProcessBuilder setEnvironmentVariable(ProcessBuilder processBuilder, Command command) {
Map<String, String> env = processBuilder.environment();
env.put(command.getEnvVarName(), command.getEnvVarContent());
processBuilder.environment().put(command.getEnvVarName(), command.getEnvVarContent());
return processBuilder;
}
}
And the method that generates the command is:
public static void createCommandToGetUrgencyList(HardCoded hardCoded) {
// create copy command
Command command = new Command();
String[] commandLines = new String[2];
commandLines[0] = "task";
commandLines[1] = hardCoded.getGetUrgencyReportName();
command.setCommandLines(commandLines);
command.setEnvVarContent("/var/taskd");
command.setEnvVarName("TASKDDATA");
command.setWorkingPath("/usr/share/taskd/pki");
command.setGetOutput(true);
// execute command to copy file
ArrayList<String> urgencyList = RunCommandsLongOutput2.executeCommands(command, false);
System.out.println("The urgency list has length = "+urgencyList.size());
for (int i = 0; i < urgencyList.size(); i++) {
System.out.println("The output of the command =" + urgencyList.get(i));
}
}
