Strings and text in Java

This page shows how to perform common string-related operations in Java using methods from the String class and related classes. In some cases, there may be more efficient ways to perform that task, but we've generally chosen the one that involves the simplest code.

String operationJava code
Represent a fixed string in your programPut the fixed string in quotes: 
"Hello!"
Print a fixed string to the console, e.g. for debugging. 
System.out.println("Hello!");
Don't rely on accented characters etc from being written properly. The Console class may allow you to output accented characters correctly.
Assign a string to a variable, then later print it 
String str = "Hello";
...
System.out.println(str);
Append strings You can generally use the + operator between strings; you can also use +=: 
String str = "Hello" + " " + "world!";
str = "Well... " + str;
str += "And goodbye!";
Note that Java Strings are immutable. So srtictly speaking, code that appears to be appending one string to another will actually be either: (a) creating a brand new string object on each "addition" (even if no-longer needed are immediately garbage collectable) or (b) compiled in such a way that a StringBuilder is actually used, and the += operations are effectively translated into calls to StringBuilder.append().
Check which character is in a given position. Use charAt(), remembering that the positions start at zero. For example: 
char firstChar = str.charAt(0);
char secondChar = str.charAt(1);
Get the last character in a string. Use charAt() again, but use the list index in the string, which is equal to the length minus one (because indexes start at zero): 
char lastChar = str.charAt(str.length() - 1);
Put a line break inside a stringPut "\n" where you want the line break: 
"Line one\nLine two";
Pop up a string in a dialog box 
JOptionPane.showMessageDialog(null, "Hello, world!");
Append an integer (int or long) to a stringFor integers, just use +: 
String msg = "Lives left: " + livesLeft;
Append a float or double to a stringTo print or append a floating point value, it's best to use String.format() to specify the number of decimal places you want to print to: 
String msg = "Value of x = " + String.format("%.3f", x);
Here, .3 specifies three decimal places.
Print/append a number padded with spaces 
String.format("% 8d", number);
Note the space before the 8; the 8 indicates the padding width.
Print/append a string padded with spaces 
String.format("% 20s", str);
Print a number in hex (convert a number to hex)Use Integer.toHexString() or Long.toHexString(): 
String msg = "Hex: " + Integer.toHexString(val);
Convert a String into an int, long, double etc. Use Integer.parseInt(), Long.parseLong(), Double.parseDouble() etc. 
String str = "12345";
int n = Integer.parseInt(str);
Print a date, or add it to a string
Format a date as DD/MM/YYYY: 
Date d = new Date();
String str = String.format("%td/%<tm/%<tY", d);
You can rearrange the elements as required; those after the first need the <.
Format a date in the "usual numeric form for the local region": 
Date d = new Date();
String str = String.format("%tD", d);
Convert a string to upper/lower case 
str = str.toUpperCase();
str = str.toLowerCase();
Test if "string contains only digits" 
if (str.matches("\\d*")) {
  ...
}
To test if the string consists of "between 5 and 10 digits": 
if (str.matches("\\d{5,10}")) {
  ...
}
For more information on how this works, see the section on regular expressions.
Test if string X equals string Y 
if (strX.equals(strY)) {
  ...
}
Note that this will throw an exception if strX is null.
Test if string X is equal to a constant string 
if ("constantString".equals(str)) {
  ...
}
Putting the constant string first in the expression prevents a NullPointerException from being thrown if str is null.
Test if string X contains string Y, taking case into account 
if (strX.contains(strY)) {
  ...
}
Test if string X contains string Y, ignoring case 
if (strX.toUpperCase().contains(strY.toUpperCase())) {
  ...
}
You could also use:
  • regular expressions, using a case-insensitive match;
  • the String.regionMatches() method inside a loop;
But you may find this version easier to understand and remember!
Limit a string to a maximum length he following ensures that str has a maximum length of 50: 
str = String.format("%50s", str).trim();
Read strings from a file 
File f = new File("C:\\TextFile.txt");
BufferedReader br = new BufferedReader(new FileReader(f));
try {
  String line;
  while ((line = br.readLine()) != null) {
    ... do something with line ...
  }
} finally {
  br.close();
}
Exception handling has been omitted for simplicity. If you're fussy about character encoding (i.e. your file contains characters other than the "standard" English unaccented letters, numbers and punctuation), then you may need to use: 
BufferedReader br = new BufferedReader(new InputStreamReader(
  new FileInputStream(f), "ISO-8859-1"));
Where ISO-8859-1 is the character encoding (the most common alternative is UTF-8).

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Editorial page content written by Neil Coffey. Copyright © Javamex UK 2021. All rights reserved.