Strings – Introduction to Programming

The String type is defined in a class named String found in a file named String.java.

The String type is a reference type meaning that a variable of type String holds a reference (memory address on the Heap) to where an instance of the String class is actually stored.

The String class provides

storage for a sequences of characters that define the String
a ton of method that allow us to retrieve information about the String and manipulate the String

We can create strings using a String literal or a constructor, or a combination of variables and literals.

String str0 = "cat";
String str1 = new String();
String str2 = new String("hat");

char[] data = {'c', 'a', 't'};
String str3 = new String(data);              // "cat"
String str4 = new String(data, 1, 2);        // "at"
 
byte[] ascii = {99, 97, 116};
String str5 = new String(ascii);             // "cat"
String str6 = new String(ascii, 1, 2);       // "at"

String str7 = str0 + " in the " + str2;      // concatenation
String str8 = new String(str7);          
String str9 = str8;

Equality

String s1 = "hello";
String s2 = s1;
String s3 = "hello";
String s4 = new String(s1);

System.out.println(s1==s2?"equal":"unequal"); 
System.out.println(s1==s3?"equal":"unequal");  
System.out.println(s1==s4?"equal":"unequal");

We can call class methods on an instance of a class using the dot operator as shown below.

The equals() method compares characters and the equalsIgnoreCase(), well, compares characters while ignoring the case of the characters. Both methods return a boolean value.

String s5 = "Hello";
System.out.println(s1==s5?"equal":"unequal"); 
System.out.println(s1.equals(s5)?"equal":"unequal"); 
System.out.println(s1.equalsIgnoreCase(s5)?"equal":"unequal");

Operations on Strings

Obtaining the Length of a String

You can find the length of a string by using the length() method.

String player = "Bob";
int len = player.length();  // 3

String Concatenation

int age = 20;
String mssg0 = "Age: " + age;

If one of the operand to the + operator is a String, the compiler converts the other operand to a String. So be careful.

String mssg1 = "Age: " + age + 1;    // "Age: 201"
String mssg2 = "Age: " + (age + 2);  // "Age: 21"

Creating New Strings From Old Strings

Strings are immutable, so you can’t modify them – but you can construct new string using various String class methods.

String str1 = "scattered".substring(6); 
String str2 = "scattered".substring(1,4); // doesn't include endIndex

String str3 = "scattered".replace('c', 'h');  // "shattered"
String str4 = "scattered".replace("sc", "ch");  // "chattered"

//trim() removes leading and trailing whitespace from a string.
String str5 = " some info ".trim();    // "some info"

//join() concatenates two or more strings separated by a delimiter.
String str = String.join(":", "one", "two", "tee"); // "one:two:tee"

Character Extraction

// Get a single character at a particular index
char ch = "hello".charAt(4);  // 'o'

// Get more than one character
String str = "hello world";
char[] buffer = new char[10];
str.getChars(6, 2, buffer, 3);  // copies "wo" to buffer[3]

// Get all of the characters
char[] buffer = str.toCharArray();

Searching Strings

String str1 = "one two tee";
String str2 = "two";

System.out.println(str1.regionMatches(0, str2, 0,  3)?"true":"false");
System.out.println(str1.regionMatches(4, str2, 0,  3)?"true":"false");

System.out.println("peanut".startsWith("pea")?"true":"false");
System.out.println("peanut".endsWith("nut")?"true":"false");

System.out.println("hello".indexOf('e'));                // prints 1 
System.out.println("hello".indexOf("lo"));               // prints 3 
System.out.println("cat in the hat".indexOf('t', 4));    // prints 7 
System.out.println("cat in the hat".indexOf("at", 4));   // prints 12