1. More about Strings

String objects are frequently used in programs. This chapter provides extra practice in using them.

Note: the String class is one of the classes that Advanced Placement Computer Science students are expected to know well. This chapter was written after the author observed an unfortunate number of mistakes involving Strings in the AP-CS 2008 examination.

Chapter Topics:

• Strings are Immutable
• Indexing Strings
• substring()
• indexOf()

Question 1:
What does the following code write?
public class ImmDemo
{
  public static void main ( String[] args )
  {
    String str = new String("I recognize the vestiges of an old flame.");
    str.substring( 16 );
    System.out.println( str );
  }
}

Source: Bradley Kjell, http://programmedlessons.org/Java9/chap45/ch45_01.html
This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 License.

2. Strings are Immutable!

Answer:
The program writes out:
I recognize the vestiges of an old flame.
The code is syntactically correct and will compile and run, but what it does might not be what the 
author intended. (The author probably intended to write out a substring of the phrase.)

Strings are Immutable!

Programmers often forget that String objects are immutable. Once a String object has been constructed, it cannot be changed. This line of code:

str.substring( 16 );

creates a new object, containing a substring of the characters of the original object. However, the original object is not changed. Since the reference variable str continues to point to the original object, the new object immediately becomes garbage. The next statement

System.out.println( str );

writes out the characters in the original object.

Question 2:
How would you modify the program so that the new substring is written?

3. "Changing" a String

Answer:
public class ImmDemo
{
  public static void main ( String[] args )
  {
    String str = new String("I recognize the vestiges of an old flame.");
    str = str.substring( 16 );
    System.out.println( str );
  }
}

"Changing" a String

A common mistake is to think "change a string", but to then attempt to change an immutable object. When you think "change a string" you need to do two things:

1. Compute a new String object.
2. Assign the reference to the new String to a reference variable.

The diagram shows how the second version of the program works. The reference variable str is first assigned a reference to the original object. Then a new object is constructed by substring(). The new reference is assigned to str. Then the println() method is called with the new reference, so the new string is written.

Question 3:
Which character corresponds to index 0 in the following string?
I recognize the vestiges of an old flame.

4. String Indexing

Answer:
Character 'I' (the beginning character of the string).

String Indexing

The beginning character of a string corresponds to index 0 and the last character corresponds to the index (length of string)-1.

The length of a string is the number of characters it contains, including spaces, punctuation, and control characters. Fill in the following table by clicking on the buttons.

The sequence \t stands for a single character, a tab character. Each tab character counts as one character (although it might be displayed on a monitor or printer using several spaces).

Be careful about the difference between an empty string, and a null reference. An empty string is an object that contains no characters, and so has length 0. A null reference means there is no object present.

Question 4:
What does the following code write?
String myString = null;

System.out.println("The length is: " + myString.length() )

5. Versions of substring()

Answer:
The program will throw a NullPointerException (and usually stop running).
Notice that there is no object because the reference variable contains null. Since there is no 
 object, there is no length() to run.

Versions of substring()

There are two versions of substring:

There are some tricky rules about the first version of the method:

1. If from is exactly equal to the length of the original string, a substring is created, but it contains no characters (it is an empty string).
2. If from is greater than the length of the original string, or a negative value, a IndexOutOfBoundsException is thrown (and for now, your program crashes).

Question 5:
What do the following statements create?
Line of code:
New String
String line = "buttercup" ;
String a = line.substring( 6 );
String b = line.substring( 0 );
String c = line.substring( 9 );
String d = line.substring( line.length() );
String e = line.substring( line.length()+1 );
String f = line.substring( -1 );

6. Play with substring()

Answer:
(a new String is created,
containing all the characters of the original)
Line of code:
New String
String line = "buttercup" ;
buttercup
String a = line.substring( 0 );
buttercup
String b = line.substring( 6 );
cup
String c = line.substring( 9 );
(an empty string is created)
String d = line.substring( line.length() );
(an empty string is created)
String e = line.substring( line.length()+1 );
EXCEPTION
String f = line.substring( -1 );
EXCEPTION

Play with substring()

String line =  ; // Enter a string here between quotes

String result = line.substring(  ) ; // Enter a single integer parameter

result: 

Run the Method

Question 6:
The empty string "" is a legitimate String object and it has the substring( int from ) method.
What are the valid values for its from parameter?
(Test your answer with the above demonstration.)

7. Two-parameter substring()

Answer:
Only one value is legitimate: 0 .

Two-parameter substring()

Question 7:
What do the following statements create?
Line of code:
New String
String line = "buttercup" ;
String a = line.substring( 0, 6 );
String b = line.substring( 6, 9 );
String c = line.substring( 0, 1 );
String d = line.substring( 0, 0 );
String e = line.substring( 5, 5 );
String f = line.substring( 4, line.length()+1 );
String g = line.substring( 6, 3 );

8. More Play with substring()

Answer:
Line of code:
New String
String line = "buttercup" ;
"buttercup"
String a = line.substring( 0, 6 );
"butter"
String b = line.substring( 6, 9 );
"cup"
String c = line.substring( 0, 1 );
"b"
String d = line.substring( 0, 0 );
""
String e = line.substring( 5, 5 );
""
String f = line.substring( 4, line.length()+1 );
EXCEPTION
String g = line.substring( 6, 3 );
EXCEPTION

More Play with substring()

String line =  ; // Enter a string here between quotes

String result = line.substring(  , ) ; // Enter integer parameters from and to

result: 

Run the Method

Question 8:
Is the following correct? What string does it create?
String str = "buttercup" ;
str = str.substring(3).substring(1,4);

9. The indexOf() Methods

Answer:
erc
The second statement works like this: 
The final string could just as easily have been created with a single call: 
str = str.substring(4,7);

The indexOf() Methods

String example = "The sea is calm to-night." ;
int location = example.indexOf( "sea" );

String example = "The sea is calm to-night." ;
int location = example.indexOf( "rats" );

Question 9:
What does the following put into location?
String example = "The sea is calm to-night." ;
int location = example.indexOf( "a" );

10. Play with indexOf()

Answer:
6
The first occurrence of the string "a" within the method's string is at index 6.

Play with indexOf()

Question 10:
What is the index of the empty string "" ?

11. Tail of a Substring

Answer:0

Tail of a Substring

String example = "The sea is calm to-night." ;
String tail = example.substring( example.indexOf( "a" ) );

String statement = "value = alpha*beta + gamma;"  ;

int loc = statement.indexOf( "=" );
if ( loc !=  )
{
  String left = statement.substring(  0,  );
  String right = statement.substring(  + 1 );
}

Question 11:
Ooops. The fragment is not completed. Fill in the blanks.

12. Second Occurrence

Answer:
    String statement = "value = alpha*beta + gamma;"  ;
    
    int loc = statement.indexOf( "=");
    if ( loc != -1 )
    {
      String left = statement.substring(  0, loc );
      String right = statement.substring( loc + 1  );
    }

Second Occurrence

String line = "alpha = beta + 23 = 99;" ;
int spot = line.indexOf( "=" );
if (  spot != -1 )
{
  String newLine = line.substring( spot+1 ) ;
  if ( newLine.indexOf("=") > 0  )
  {
    System.out.println( "Possible Bad Line: " + line ); 
  }
}

The statement

  String newLine = line.substring( spot+1 ) ;

computes the tail of the original line, starting with the character after the first = sign. The clause newLine.indexOf("=") > 0 detects = anywhere but at the beginning of this new line (where it would be OK because it would be part of "==").

Question 12:
What if you did not have the startsWith(String str) method? Could you use other methods to replace it?

13. End of the Chapter

Answer:
line.startsWith(str)
could be replaced by
(line.indexOf(str)==0)

End of the Chapter

You have reached the tail end of the chapter. Click on a subject that interests you to go to where it was discussed.

• Strings, immutable Immutable strings
• string indexing String Indexing
• substring() substring()
• indexOf() indexOf()