• Develop code to create string objects and determine the result of creating and combining strings
  • Develop code to call methods on string objects and determine the result of calling these methods

Assignment

String References

A String is an object in Java, which is why it’s type starts with a capital letter. They’re variables are reference variables, meaning they point to the object. If it points at nothing, then it points to null.

Also, since it’s an object, it can be created like other objects.

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String s = new String("Some words");

Is just as effective as the way you are used to.

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String s = "Some words";

There are some under-the-hood differences, but nothing we need to worry about at this point.

Concatenation

You’ve seen this already, but multiple String objects can be joined with the + operator.

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String s = "Look, " + "concatenation.";

This is useful for entering the value of a variable into a String, or even numbers themselves. As long as one of the pieces is a String, then Java will convert the rest and concatenate them.

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String s = "12" + 3;    //outputs 123
String s = 12 + "3";    //also outputs 123
String s = 12 + 3;      //syntax error

String Length, Index, and Methods

Strings are made up of characters—which includes letters, numbers, symbols, and even non-printable characters like tabs and newlines—and the length of a String is how many characters it contains. Spare yourself some counting and use the length() method to get the length of a String. This is an object method, so use the dot operator.

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String s = "some string";
int l = s.length();

System.out.println(l);      // outputs 11

Each character in a String can be referenced by its index, but the counting starts at 0 (which happens a lot in programming). So, the character as position 0 is s and the one at position 3 is e. The method indexOf(String str) returns the index of where a given string can be found in a larger one.

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String s = "some string";
int i = s.indexOf("ring");

System.out.println(i);      // outputs 7

If it can’t find it, it returns -1. This is a common tactic of return methods. They are forced to return a certain type, but in the event it fails, it returns bad data rather than crashing out. Here, -1 is perfect because it’s always an invalid index.

The substring(){. header-tt} Method

Aside from length() and indexOf(), there are a few other methods you need to worry about. The method substring() is fairly straightforward on the surface, but requires practice to really wrap your head around it. Both work on an existing string and extract part of it.

There are two versions: substring(int from) and substring(int from, int to). The former returns a String starting at the index given and goes right to the end.

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String s = "some string";
String u = s.substring(6);  // "tring"

The latter includes an endpoint, but like Math.random(), the endpoint is not included.

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String s = "some string";
String u = s.substring(0, 3);  // "som"

Lots of practice needed for this, and you will see some funky problems involving it.

equals(){. header-tt} and compareTo(){. header-tt}

This came up briefly earlier, but you can compare values in Java with ==. But this is really only for primitives, because reference variables technically point to memory addresses. So, in order to properly compare objects, including Strings, the equals() method exists. Like the others, it requires the dot operator.

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String s1 = "a string";
String s2 = "a string";
String s3 = "string";

boolean s12 = s1.equals(s2);    // true
boolean s13 = s1.equals(s3);    // false

compareTo() is a bit more nuanced. It operates the same way, but returns an int. If the strings are equal, it returns 0. If the first comes first alphabetically (or more accurately, lexicographically), then it returns a negative number. It returns a positive if the second comes first.

It might seem a bit arbitrary, but think of it as a subtraction problem where the second is being subtracted from the first.

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String s1 = "apple";
String s2 = "bolognese";
int answer = s1.compareTo(s2);  // returns a negative