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String Methods

layout: default title: String Method

Because text is so important, every Java developer is expected to develop thorough knowledge of the String class and its methods.

Drill

Go to the Javadoc for class String and reference the methods we cover here: * https://docs.oracle.com/javase/8/docs/api/java/lang/String.html

String alphabet = "abcdefghijklmnopqrstuvwxyz";

Getting information about a String

  • length

System.out.println(alphabet.length()); // 26
* equals, equalsIgnoreCase

System.out.println(alphabet.equals("AbcDefGhiJklMnoPqrStuVwxYz")); // false
System.out.println(alphabet.equalsIgnoreCase("AbcDefGhiJklMnoPqrStuVwxYz")); // true
* startsWith, endsWith

System.out.println(alphabet.startsWith("abc")); // true
System.out.println(alphabet.endsWith("qrs")); // false
* contains - if another string occurs in this string, return false.

System.out.println(alphabet.contains("lmnop")); // true
System.out.println(alphabet.contains("fed")); // false
  • indexOf - if another string occurs in this string, return where it occurs (otherwise return -1).
System.out.println(alphabet.indexOf("ghi")); // 6
System.out.println(alphabet.indexOf("giraffe")); // -1

Getting at the char values in a String

The first character of a String is at position 0.

  • charAt - retrieves a single char from a certain position in the String

System.out.println(alphabet.charAt(9)); // What will this print?
for (int i = 0; i < alphabet.length(); i++) {
  char c = alphabet.charAt(i);
  if ( c % 2 == 0 ) { // even-numbered character codes.
    System.out.println(c);
  }
}
* toCharArray - copies the String contents into a new char array

char[] alphaChars = alphabet.toCharArray();
System.out.println(alphaChars[5]); // What will this print?
for (int i = 0; i < alphaChars.length; i++) {
  char c = alphaChars[i];
  if ( c % 3 == 0 ) { // Every third character.
    System.out.println(c);
  }
}
  • getChars - copies a region of the String into an existing char array.
char[] letters = new char[5];         // room for 5 chars.
alphabet.getChars(5, 10, letters, 0); // copy five chars from alphabet into the
                                      // letters array, starting at letters[0]
System.out.println(letters);

Creating transformed String values

Once created, a String is immutable, so methods manipulating String contents always return a newly-created String, leaving the original unchanged.

  • concat - create a new String from the original and the argument.
String hello = "Hello";
String helloWorld = hello.concat("World");
System.out.println(helloWorld); // HelloWorld
System.out.println(hello); // Hello

  • Because we can just use + we seldom use concat.

  • toUpperCase, toLowerCase - create an all-uppercase or all-lowercase copy of the String.

System.out.println(alphabet.toUpperCase());
// ABCDEFGHIJKLMNOPQRSTUVWXYZ
System.out.println(alphabet);
// abcdefghijklmnopqrstuvwxyz
  • trim - remove whitespace from the beginning and ending of a string.

String trimmed = "\t   hello world \n".trim();
System.out.println(trimmed.equals("hello world")); // true
* For trim, "whitespace" means any character from Unicode \u0000 through "", including tabs, newlines, and control characters. * Spaces inside the string (between the first non-whitespace character and the last) are left alone.

  • split - break a string into pieces based on a delimiter in the string's value.
String[] pieces = alphabet.split("m");
// ["abcdefghijkl", "nopqrstuvwxyz"]

  • The delimiter itself is discarded.

    String[] fields = "cat, dog, frog".split(", "); // split on ", "
// ["cat", "dog", "frog"]

String Indexes

  • substring

  • The first index is the zero-based starting point.

  • The second index is the index after the end of the range.

    • If you omit the second index, substring returns the text from the first index to the end of the string.

// 0123456789
// abcdefghijklmnopqrstuvwxyz
String sub1 = alphabet.substring(4, 8);
System.out.println(sub1); // efgh
String sub2 = alphabet.substring(4);
System.out.println(sub2); // efghijklmnopqrstuvwxyz
* That is, the first index is inclusive, the second index is exclusive.

Drill

StringAndStringBuilder/src/com.example.stringstringbuilder.drills.SSNMasker (src source folder)

StringAndStringBuilder/test/com.example.stringstringbuilder.drills.SSNMaskerTests (test source folder)

  1. Run SSNMaskerTests as a JUnit Test. It will fail.
  2. In SSNMasker write the implementation of the maskSSN method, using substring and concatenation, so that the JUnit tests pass.

Converting a Primitive to a String

String provides a valueOf method for generating the String representation of any primitive type.

float f = 1.214f;
String fStr = String.valueOf(f);
System.out.println(fStr); // "1.214"
boolean b = true;
String bStr = String.valueOf(b);
System.out.println(bStr); // "true"

Because String is special, you can use concatenation as a shortcut:

System.out.println("" + 1.214); // "1.214"
System.out.println("" + true); // "true"
System.out.println("" + null); // "null"

Practice Exercise

The + concatenation operator only works when one of the operands is a String. ```java System.out.println(null + true); // WILL NOT COMPILE System.out.println(true + null); // WILL NOT COMPILE

System.out.println(null + null); // WILL NOT COMPILE ```

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