Strings

In java, Strings are a sequence of characters. You might have noticed that in Java, a string is not a primitive data type, and is initialized using the String class. You'll also notice that in the rules, you must know the following methods: length, charAt, getChars, equals, equalsIgnoreCase, startsWith, endsWith, indexOf, lastIndexOf, substring, and concat. You will also need to know string concatenation using the + operator. I will be going over each method in detail.

Before I go into the methods, let's briefly look into how to instantiate a string:

String a = "this is a string";                    // "this is a string"
String b = new String("this is another string");  // "this is another string"
char[] c = {'s', 't', 'r', 'i', 'n', 'g', 's'};
String d = new String(c);                         // "strings"

As you can see, there are multiple ways to instantiate a string; you can even use a character array.

length

This method is very self explanatory. You basically get the length of a string. Let's see it's usage below:

String a = "a string";
int a_length = a.length(); // 8

String b = "";
int b_length = b.length(); // 0

Exercises

1. What is the result of "hello world".length() ?
2. What is the result of "this is a str".length() ?
3. What is the result of "coding".length() ?

charAt

This method gets the character at a certain index of a string. Let's see it's usage below:

String a = "a string";
char b   = a.charAt(0); // 'a'
char c   = a.charAt(6); // 'n'

Note that this method doesn't work at all on an empty string.

Exercises

1. What is the result of "hello world".charAt(6) ?
2. What is the result of "this is a str".charAt(11) ?
3. What is the result of "coding".charAt(3) ?

getChars

This method is a bit more complicated. The syntax of this method is below:

public void getChars(int srcBegin, int srcEnd, char[] dst,  int dstBegin)

srcBegin is the index of the first character in the string to copy

srcEnd is the index after the last character in the string to copy

dst is the character array you copy the characters to, aka the destination array

dstBegin is the start offset in the destination array (i.e. the index where you begin copying the characters)

This method is probably one of the most confusing methods to figure out, so I'll go through some examples to make things clear.

String str1 = "Code Analysis!";
char[] str2 = new char[8];

str1.getChars(5,13,str2,0);

This table should explain how str1 is structured:

As you can see in the code block, we use getChars on str1, and our starting index is 5 , so we INCLUDE the A character, and our ending index is 13 , but we EXCLUDE the ! character, so our full string that we extract becomes "Analysis" . Our source character array is also of length 8 (by the way make sure your destination array can fit the string you extract). We set our offset to 0 , because we want to start copying characters from the beginning of the array. Thus, the character array now looks like this:

str2 = {'A', 'n', 'a', 'l', 'y', 's', 'i', 's'};

Exercises

For these exercises, I will give them in code blocks

// 1.
String str1 = "Code Analysis!";
char[] str2 = new char[4];

str1.getChars(0,4,str2,0);

// What is the value of str2: {_, _, _, _} ?

// 2.
String str1 = "Does anyone remember CodeBusters 2014?";
char[] str2 = new char[8];

str1.getChars(21,25,str2,0);

// What is the value of str2: {_, _, _, _, _, _, _, _} ?

// 3.
String str1 = "The States test will be really hard";
char[] str2 = new char[8];

str1.getChars(11,15,str2,4);

// What is the value of str2: {_, _, _, _, _, _, _, _} ?

equals

This method checks if two strings are equal. Let's see it's usage below:

String a = "this";
String b = "that";
String c = "this";

boolean d = a.equals(b); // false
boolean e = a.equals(c); // true

Exercises

1. What is the result of "hey".equals("hello") ?
2. What is the result of "what".equals("what") ?
3. What is the result of "this is tedious".equals("This is tedious")?

equalsIgnoresCase

This method checks if two strings are equal, BUT it ignores case. Let's see it's usage below:

String a = "this";
String b = "This";
String c = "THIS";
String d = "that";

boolean e = a.equals(b); // true
boolean f = a.equals(c); // true

Exercises

1. What is the result of "what".equals("whAt") ?
2. What is the result of "this".equals("th1s") ?
3. What is the result of "whoa".equals("WHOA") ?

startsWith

This method actually has two ways of being used: boolean startsWith(String str), and boolean startsWith(String str, index fromIndex). The first way simply checks if a string starts with str, and the second way checks if a string starts with str from a specified index in the original string. Let's see how this method is used below:

String str1 = "Code Analysis is great";
String str2 = "Code";

boolean a = str1.startsWith(str2);    // true

String str3 = "This is a string";
String str4 = "is";

boolean b = str3.startsWith(str4, 5); // true

Exercises

1. What is the result of "the quick brown fox".startsWith("the") ?
2. What is the result of "the quick brown fox".startsWith("brown", 4) ?
3. What is the result of "the lazy dog".startsWith("dog", 9) ?

endsWith

This method is very similar to startsWith, except it only has one way of being used: boolean endsWith(String str) . This method checks if a string ends with str . Let's see how this method is used below:

String str1 = "Code Analysis is great";
String str2 = "great";

boolean a = str1.endsWith(str2); // true

String str3 = "Code Analysis is awful";
String str4 = "amazing";

boolean b = str3.endsWith(str4); // false

Exercises

1. What is the result of "the quick brown fox".endsWith("the") ?
2. What is the result of "the quick brown fox".endsWith("fox") ?
3. What is the result of "rats live on no evil star".endsWith("star") ?

indexOf

The indexOf method searches a string str from left to right for the given string target if you run it as such: str.indexOf(target) . Keep in mind that if it doesn't find the string, it returns -1 . Let's look at an example of this below:

String str = "Here there everywhere";

int a = str.indexOf("there"); // 5
int b = str.indexOf("er");    // 1
int c = str.indexOf("eR");    // -1

The indexOf method also has behavior similar to startsWith, in that you can provide an index as the second argument, and it will start searching the string from that index. The method definition looks like this: int indexOf(String str, int index) . Let's look at an example of this below:

String str = "this is this, and that is that";
int a = str.indexOf("this", 4);  // 8
int b = str.indexOf("that", 22); // 26

Exercises

1. What is the result of "this is a regular string".indexOf("is") ?
2. What is the result of "this is a regular string".indexOf("regular") ?
3. What is the result of "how do you do".indexOf("o", 6) ?

lastIndexOf

The lastIndexOf method is similar to the indexOf method, except it returns the last instance of what you're looking for. It also has functionality where you can start searching _backward _from a given index. This method also returns -1 when it doesn't find the string. Let's look at how it's used below:

String str = "this is this, and that is that";

int a = str.lastIndexOf("this");    // 8
int b = str.lastIndexOf("this", 4); // 0
int c = str.lastIndexOf("these");   // -1

Exercises

1. What is the result of "this is the last".lastIndexOf("s") ?
2. What is the result of "this is the last".lastIndexOf("t", 8) ?
3. What is the result of "how now brown cow".lastIndexOf("ow") ?

substring

The substring method in java has two ways to use it: String substring(int beginIndex) and String substring(int beginIndex, int endIndex) . The first way takes the string from the beginning index all the way to the end of the string. The second way takes the string from the beginning index to the character before the end index. Let's look at how it's used below:

String str = "horsecatdogpigeonliontigerpig";

String catstr = str.substring(5, 8);      // "cat"
String pigeonstr = str.substring(12, 17); // "pigeon"
String pigstr = str.substring(26);        // "pig"

Exercises

1. What is the result of "this is a regular string".substring(10) ?
2. What is the result of "this is a special string".substring(10, 17) ?
3. What is the result of "this is a boring string".substring(0, 4) ?

concat

The concat method in java takes two strings, and sticks them together, or "concatenates" them. It works like this: String concat(str) . Note: the + operator with strings does EXACTLY the same thing as concat for our purposes. There are a few gotchas I'll go over at the end of the lesson. Let's look at some examples below:

String leftHandSide = "this is ";
String rightHandSide = "a string";

String combined = leftHandSide.concat(rightHandSide); // "this is a string"

Here are the main differences of + and concat :

null + "some string"       // "nullsome string"
null.concat("some string") // NullPointerException

Exercises

1. What is the result of "cat".concat("con") ?
2. What is the result of "Virginia".concat("Tech") ?
3. What is the result of "how do you".concat(" concatenate strings") ?
4. What is the result of "how do" + " you do" ?
5. What is the result of "+" + "+" ?
6. What is the result of "this is " + "a string" ?