Java Tutorial for Python Programmers

Java Strings

Python has lots of notation that can be used directly on strings, that Java does via methods. The exception to this is concatenation, which we've already seen (and which Java does better by automatically applying toString to non-strings). I'll cover the other basics here. For a complete reference for Java string methods, check out the Java 7 String API.

String.length

len("monkey") becomes "monkey".length(). There is no magical len function that returns the length of any iterable object. Instead, we use the length method for strings. Here's an example in an updated main method: (the \" is the escaped double-quotes character)

    public static void main(String[] args) {
        
        System.out.println("Hello, World!");
        String simian = "monkey";
        System.out.println("\"monkey\" has " + simian.length() + 
                           " characters.");
    }

Compile and run this to make sure you've got it working.

String.charAt

"monkey"[3] becomes "monkey".charAt(3). We can use square brackets with Java arrays, but not strings. (Sorry!) To get a specific character, use the charAt method. Here's an example:

    public static void main(String[] args) {
        
        System.out.println("Hello, World!");
        String simian = "monkey";
        System.out.println("The foureth character of \"monkey\" is " + 
                           simian.charAt(4) + ".");
    }

Compile and run this code.

String.substring

"monkeybingo"[3:8] becomes "monkeybingo".substring(3, 8). Slices don't work at all in Java. For strings, we use the substring method instead. As always, here's an example of the method in action:

    public static void main(String[] args) {
        
        System.out.println("Hello, World!");
        String food = "pearstrawberry";
        System.out.println("There are " + food.substring(1, 5) + 
                           " in my " + food + "!");
    }

If you compile and run this, it should say: There are ears in my pearstrawberry!

No negative indices.

Java's indices run from 0 to the length of the string (or list) minus 1. Try running the following code to see what happens if the index is out of these bounds:

    public static void main(String[] args) {
        
        System.out.println("Hello, World!");
        String food = "pearstrawberry";
        System.out.println(food.charAt(-3));
    }

Running this leads to the following:

$ javac *.java
$ java CodeToRun
Hello, World!
Exception in thread "main" java.lang.StringIndexOutOfBoundsException:
String index out of range: -3
	at java.lang.String.charAt(String.java:646)
	at CodeToRun.main(CodeToRun.java:16)

String.indexOf

"monkey".find("n") becomes "monkey".indexOf("n"). The Java method is overloaded and works both with strings and characters as parameters. Check out the following code as an example:

    public static void main(String[] args) {
        
        System.out.println("Hello, World!");
        String band = "bananarama";
        System.out.println("Should be 4: " + band.indexOf("nar"));
        System.out.println("Should be 6: " + band.indexOf('r'));
        System.out.println("Should be -1: " + band.indexOf("x") + 
                           "    -1 means it doesn't contain the parameter.");
    }

String.contains

"key" in "monkey" becomes "monkey".contains("key"). This returns a boolean exactly as you would expect. Try out this code:

    public static void main(String[] args) {
        
        System.out.println("Hello, World!");
        String band = "bananarama";
        System.out.println("Has \"banana\": " + band.contains("banana"));
        System.out.println("Has \"apple\": " + band.contains("apple"));
    }

String.equals

"monkey" == animal becomes "monkey".equals(animal). In Java, a String is a type of object. You can still use ==, but it won't do what you expect. Instead of comparing the values in the expression x == y, Java just tests whether the two object variables point to the same memory address. While this will always work for primitive values (ints, booleans, floats, doubles) it won't work for two objects that have the same value but aren't stored in the same place. Compile and run this code:

    public static void main(String[] args) {
        
        System.out.println("Hello, World!");
        String band = "bananarama";
        String sameBand = band; //band and sameBand are stored in the same place.
        System.out.println("These two are equal; the test works:" + (band == sameBand));
        System.out.println("These two are equal; better test:" + (band.equals(sameBand)));
        String sameBandAgain = new String("bananarama"); //stored in a new place
        System.out.println("These two are equal; test fails:" + (band == sameBandAgain));
        System.out.println("These two are equal; test works:" + (band.equals(sameBandAgain)));
    }

It's very common for beginner Java programmers to use == to test the equality of non-primitives. This isn't an issue in Python, because == automatically calls the special __cmp__ method that should test for equality. If something is going wrong, take a look at whether you're using the equals method less often than you should be.

Test Your Understanding

Rewrite your printMonkey method so that there is only one print statement. You'll probably have to use some of the string methods and some other variables to do this. I recommend using substring with a variable for one of the arguments. (That variable can change depending on whether the monkey is funny or not.) Check that both of the calls to printMonkey still work correctly.