Types in Java

1. Types in Java Computer Science 3 Gerb Objective: Understand type declarations in Java

2. Local Variables • The programmer can store data inside a method. • Called a local variable • Disappear when the method is finished running. • To declare a local variable: • The type • Followed by the variable name • Followed by a semicolon: int myInt; • Variable names should be descriptive • Never pass up an opportunity to make your code more readable • Bad: a, b, dog, pete • OK: biggest, myTable, inputVal

3. We will study 5 major data types • int - An integer • boolean - Can be true or false • double - Can hold decimal numbers • String - Holds a sequence of characters • Denoted by characters inside quotes (“”) • class

4. Giving values to variables • Variables can be given values when declared: int myInt=6; boolean myBool=false; String myString = “Some characters”; • Variables can be given values through an assignment statement: myDouble=1.0027; myString = “This is a string”; • Variable name, followed by equal sign, followed by value. • Multiple variables can be declared on the same line int int1=2, int2, int3=4;

5. Constants • It is possible to store a value in a variable that cannot change. • Precede the declaration by the word final: final int myConst=4488; • Must initialize the variable in the declaration • Can’t ever assign a value to a final variable • In Java a final variable is called a constant • Useful for defining a value that will be used throughout a method but will never change while that method runs.

6. More about Strings • Strings are typically set to a sequence of characters within double quotes • Some characters are awkward to place between double quotes • New line character: Use a backslash (\) followed by n • Double quote character: Use a backslash followed by a double quote (\”) • Backslash character: Use two backslashes (\\) • Example, ends with a backslash, a quote, then a new line: “A string\\\”\n”

7. Plus sign concatenates strings Concatenating Strings • Given two strings, can make a new string made up of the first followed by the second. • Called concatenating strings • Use the plus sign String s1 = “abc”; String s2 = “def”; String s3 = s1 + s2; System.out.print(s3); //prints abcdef • Two quotes with nothing in between (“”) is called the empty string • Concatenating a string with the empty string equals itself

8. Can use concatenation to break long lines • Hard to read (indentation is fouled): public static void main(String[] args){ System.out.println(“This line is too long to fit”); } • Easier to read: public static void main(String[] args){ System.out.println(“This line is ” + “too long to fit”); }

9. Summary • Variables declared inside methods are called local variables. • Type, followed by variable name and semicolon • Java supports int, double, boolean, String and user defined classes • Strings can be set to characters inside double quotes • Use backslash for special characters • Use plus sign to concatenate two strings