[Solved] How to writer code for File Handling Problem

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First you will need a integer variable to hold the size of string you want to build. You initially gave an example of 500 initialize the variable with that amount:

// Desired length of string to create from file contents.
int desiredStringLength = 500;

Depending upon what is supplied to the desiredStringLength variable will determine what the MINIMUM allowable character content that can be within the supplied text file for the code to be successful in building the string. This is required since to build the string only a single acquisition is allowed from the beginning of file (according to your post) and there must be enough characters within the file to take care of business. This Minimum Allowable file content value is determined by taking the supplied desiredStringLength and dividing it by 2. If however the division leaves a remainder then 1 is added to the division calculation:

/* Calculate and hold the MINIMUM number of characters 
   allowed to be in the supplied text file. A Ternary
   Operator is used to select the appropriate calculation.  */ 
int minimumFileLength = (desiredStringLength % 2 == 0 ? 
                         desiredStringLength/2 : 
                         (desiredStringLength/2) + 1);

Now you just need to get the contents of the text file and place its contents into a single string variable:

String fileAsString = ""; // Holds all the file data as a single string.
try {    
    fileAsString = new String(Files.readAllBytes(Paths.get("test2.txt")));
}
catch (IOException ex) {
    // Inform of any errors then exit method
    System.err.println("File Error! --> " + ex.getMessage());
    return;
}

Now the fileAsString string variable holds the contents of the text file as a single string. It’s now just a matter of pulling out the characters you want with whatever methods you like. The entire code might look something like this:

String ls = System.lineSeparator(); // OS dependent 'System Line Break' character(s).
int desiredStringLength = 500;      // Desired length of string to create from file contents.

/* Calculate and hold the MINIMUM number of characters 
   allowed to be in the supplied text file. A Ternary
   Operator is used to select the appropriate calculation.  */ 
int minimumFileLength = (desiredStringLength % 2 == 0 ? 
                        desiredStringLength/2 : 
                        (desiredStringLength/2) + 1);

String fileAsString = ""; // Holds all the file data as a single string.
String finalString = "";  // Will hold the string which will be built.

// Display some information...
System.out.println("Desired length of String to create from File: --> " + desiredStringLength);
System.out.println("Minimum 'Allowable' Character count in file: ---> " + minimumFileLength);

// Place the entire file contents into the fileAsString variable.
try {    
    fileAsString = new String(Files.readAllBytes(Paths.get("test2.txt")));
}
catch (IOException ex) {
    // Inform of any errors then exit method
    System.err.println("File Error! --> " + ex.getMessage());
    return;
}

// More informational display...
System.out.println("Actual character count in file: ----------------> " + fileAsString.length());
System.out.println(ls + "The Created String:");
System.out.println("===================");

// If the file contents is less the the minimum allowable then 
// Inform User and exit method.
if (fileAsString.length() < minimumFileLength) {
    System.err.println("File Content Error! The supplied file does not " +
                       "contain enough content to build a string " + desiredStringLength + 
                       " characters long!" + ls + "File content " + "total: --> " + 
                       fileAsString.length() + " characters");
    return;
}
/* If the file content count is less than the desired build 
   string length then subtract what is contained in file from 
   the desired string build size and pull the calculated amount 
   from the beginnig of file content and append it to the file 
   content. This is all placed into the finalString variable.  */
else if (fileAsString.length() <= desiredStringLength) {
    finalString += fileAsString + fileAsString.substring(0, desiredStringLength-fileAsString.length());
}
/* If the file content count is at the desired string build size
   or greater then just build the desired string from the beginning
   of the file content.       */
else {
    finalString = fileAsString.substring(0, desiredStringLength);
}

// Display the built string.
System.out.println(finalString);
// Inform User of built string length.
System.out.println(ls + "Created String Length: --> " + finalString.length());

solved How to writer code for File Handling Problem