1.15 String Manipulation

1. Exam Points

• Manipulate strings using methods
  • int length().
  • String substring(int from, int to)
  • String substring(int from)
  • int indexOf(String str)
  • boolean equals(Object other)
  • int compareTo(String str)
• Note:
  • index of a string starts from 0
  • get one character from a string: substring(index, index + 1).
  • get n characters from a string: substring(index, index + n).
  • get BCD from ”ABCD.EF”: substring(1, 4)
  • check if two strings are equal: use the equals() method.

2. Knowledge Points

(1) The String Class

• A String object represents a sequence of characters and can be created by using a string literal or by calling a String class constructor.
• The String class is part of the java.lang package. Classes in the java.lang package are available by default.
• A String object is immutable (不可变的), meaning once a String object is created, methods called on a String object do not change the content of the String object.
  
  • here we get a new string str2 by manipulating str1, but str1 remains unchanged.
• Two String objects can be concatenated together or combined using the + or += operator, resulting in a new String object.
• Use () to avoid mistakes. c += a + b is c = c + (a+b)

(2) String Manipulation

• A String object has index values from 0 to length-1.
• Attempting to access indices outside this range will result in a StringIndexOutOfBoundsException.
• A string identical to the single element substring at position index can be created by calling substring(index, index + 1).
  • Example:
  • String str = "ABCDEF";
  • String newStr = str.substring(2,3)
  • newStr is C
• String methods:
  • int length(): returns the number of characters in a String object.
  • String substring(int from, int to): returns the substring beginning at index from and ending at index to - 1.
  • String substring(int from): returns substring(from, length()).
  • int indexOf(String str): returns the index of the first occurrence of str; returns -1 if not found.
  • boolean equals(Object other): returns true if this corresponds to the same sequence of characters as other; returns false otherwise.
  • int compareTo(String other): returns a value < 0 if this is less than other; returns zero if this is equal to other; returns a value > 0 if this is greater than other. Strings are ordered based upon the alphabet.
• Example:

  
    String str = "ABCDEFG";
    
    // 1. length()
    int len = str.length();
  
    // 2. indexOf() : int indexOf(String str)
    int index1 = str.indexOf("A"); // result: 0
    int index2 = str.indexOf("D"); // result: 3
    int index3 = str.indexOf("EF"); // result: 4
    int index4 = str.indexOf("G"); // result: -1
  
    // 3. substring(int from)
    String substring1 = str.substring(2); // result: BCDEFG
    String substring2 = str.substring(4); // result: EF
  
    // 4. substring(int from, int to)
    String substring3 = str.substring(3, 5); // result: DE
  
    // 4.1 get one character
    String email = "annabelle.li@qq.com";
    String substring4 = email.substring(0, 1); // result: a
  
    // 4.2 get two characters
    String substring5 = email.substring(0, 2); // result: an
  
    // 4.3 get three characters 
    String substring6 = email.substring(3, 6); // result: abe
  
    // 5. equals() : boolean equals(Object other)
    String str1 = "lily";
    String str2 = new String("lily");
    System.out.println(str1.equals(str2)); // true
    System.out.println(str2.equals(str1)); // true
  
    // 6. compareTo(String str)
    String x = "abc";
    String y = "axc";
    // the returned value indicates which string is larger
    // 0: x equals y
    // negative: x less than y alphabetically
    // positive: x greater than y alphabetically
    int result = x.compareTo(y); // result: a negative integer
    
  

3. Exercises

• Practice
• Solution