String Literals

A string literal consists of zero or more characters enclosed in double quotes; for example:

"hello"
"416-736-2100"
"computer science and engineering"

Weirdness Part I

A string literal is actually a String object. You might find this surprising because an object is created without using a constructor or calling a method.

import java.io.PrintStream;

public class StringLiteral
{
   public static void main(String[] args)
   {
      PrintStream output = System.out;

      // length of a string      
      output.print("number of characters : ");
      output.println("hello".length());
      
      // get the character in position 4
      //   counts from 0
      output.print("fourth character : ");
      output.println("hello".charAt(4));
      
      // same state?
      output.print("same state : ");
      output.println("hello".equals("hello"));
   }
}

number of characters : 5
fourth character : o
same state : true

Weirdness Part II

A string literal is a reference to a String object; everytime you use a particular literal, for example "hello", you are referring to the same object. The code snippet:

      // same object?
      output.print("hello" == "hello");
      
      output.print(" ");      
      
      // same object?
      String s = "hello";
      output.print(s == "hello");

will print true true.

Memory Diagram I

Consider the following program:

public class StringMemoryDiagram1
{
   public static void main(String[] args)
   {
      char first = "York University".charAt(0);
   }
}

Its memory diagram looks like:

Memory Diagram II

Consider the following program:

public class StringMemoryDiagram2
{
   public static void main(String[] args)
   {
      char first = "York University".charAt(0);
      String york = "York University";
   }
}

Its memory diagram looks like:

Weirdness + Memory Diagram III

A String object can also be created using new; new always creates a new object.

public class StringMemoryDiagram3
{
   public static void main(String[] args)
   {
      char first = "York University".charAt(0);
      String york = "York University";
      
      String s = new String("York University");
      String t = new String(york);
   }
}

Its memory diagram looks like:

The objects referred to by s and t are wasteful; we already have a variable named york that has the desired state. You will see in the next lecture that a client cannot change the state of a String object.

Weirdness Part IV

You can join two strings to create a new string using the concatenation operator +:

      String name1 = "James Bond 007";
      String name2 = "James" + " " + "Bond" + " " + "00" + 7;

      String firstName = "James";
      String lastName = "Bond";
      String number = "007";
      String name3 = firstName + " " + lastName + " " + number;

1. name1 is a reference to a String literal
2. name2 is a reference to a String computed using an expression using only literals and +
   • such an expression is called a constant expression
   • a constant expression is treated as though it were a literal
   • name1 == name2 is true
3. name3 is a reference to a String computed using an expression using variables, literals, and +
   • a new String object is created for all such expressions
   • name1 == name3 is false

Weirdness Part V

Concatenation and addition both use + as the operator. This can lead to errors if you forget that Java evaluates expressions involving operators of the same precedence from left to right.

      String laugh = 'h' + 'a' + "ha";
      System.out.println(laugh);

The above snippet prints:

201ha

Weirdness Part V Explained

      String laugh = 'h' + 'a' + "ha";
      System.out.println(laugh);

The expression 'h' + 'a' + "ha" involves operators all having the same precedence, so it is evaluated from left to right:

1. 'h' + 'a' is the sum of two char literals; addition is not defined for char so both operands are promoted to int and then summed to yield 201
2. 201 + "ha" involves a String object so string concatenation is performed to yield "201ha"

You can fix the problem by concatenating an empty string "" at the beginning of the expression:

      String laugh = "" + 'h' + 'a' + "ha";

Weirdness Part VI

You can also concatenate a string with an object reference. The object reference is converted to a String by calling its toString method.

      Fraction f = new Fraction(5, 8);
      String s = "The fraction is " + f;
      System.out.println(s);

The above code snippet prints:

The fraction is 5/8

Weirdness VII

What does the following print?

      String s1 = "Julie Payette";
      String s2 = "Julie " + "Payette";
      System.out.println("same object: " + s1 == s2);

To Do For Next Lecture

Continue reading Chapter 6.