c_5_2_reviewingStringClass

🧠 5.2 Reviewing String Class

• The String class is final and String objects are immutable.

🔴 String Equality

• In order to see if two strings are equal in content, you must use .equals() rather than == which checks for reference equality.
• E.g.

String s1 = "bunny";
String s2 = "bunny";
String s3 = new String("bunny");
s1 == s2; // true as string pool can reuse reference
s1 == s3; // false - using constructor creates new String in pool
s2 == s3; //false - same as above

🔴 String Concatenation

• String concatenation is achieved using + operator. The result creates a brand new String with the two strings concatenated.

String s4 = "1" + 2 + 3; // 123
String s5 = 1 + 2 + "3"; // 33

• Here are some common String methods:

String s = "abcde";
s.trim().length(); // 5
s.charAt(4); // e
s.indexOf('e'); // 4
s.indexOf("de"); // 3
s.substring(2,4).toUpperCase(); // CD
s.replace('a', '1'); // 1bcde
s.contains("DE"); // false
s.startsWith("a"); // true

🔴 StringBuilder

• StringBuilder is mutable, but not thread safe.
• StringBuffer is also mutable, slower but thread safe.

StringBuilder b = new StringBuilder();
b.append(12345).append('-'); // 12345-
b.length();                  // 6
b.indexOf("-");              // 5
b.charAt(2);                 // 3
StringBuilder b2 = b.reverse(); 
b.toString(); // -54321
b == b2; // true

• The StringBuilder class does not override the equals() method. So we can only check for reference equality!

StringBuilder s = StringBuilder("abcde");
s.insert(1, '-').delete(3,4) // a-bcde -> a-bde
s; // a-bde
s.substring(2,4); // bd