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Unlike Python, Java variables must have their type declared before an assignment can be made. Try changing your main method to add the line from above: (I also removed the part about bananas.)
public class CodeToRun {
public static void main(String[] args) {
System.out.println("Hello, World!");
simian = "monkey";
}
}
Now try compiling and running your code. You should see a message something like the following:
$ javac *.java
CodeToRun.java:6: error: cannot find symbol
simian = "monkey";
^
symbol: variable simian
location: class CodeToRun
1 error
In order to turn this into working Java code, we have to tell the program what type the variable simian will have, called declaring the variable. The following is a modified version of the body of the main method, with a declaration statement added:
System.out.println("Hello, World!");
String simian; //declaration for simian
simian = "monkey"; //assignment to simian
This code will compile and run happily:
$ javac *.java $ java CodeToRun Hello, World!
We can also combine those two into a single statement, known as an initialization:
System.out.println("Hello, World!");
String simian = "monkey"; //initialization for simian
Initializations are more common than sole declarations, but there are some times where you might want to declare a variable without giving it a value.
We can use the variable as part of a print statement:
System.out.println("Hello, World!");
String simian = "monkey"; //initialization for simian
System.out.println("simian: " + simian);
$ javac *.java $ java CodeToRun Hello, World! simian: monkey
Other common types are int, boolean, float, and double. These last two are similar to Python's float type, except that double keeps track of twice as many significant digits. These are all primitive types in Java, meaning that they are not stored as objects and thus have no associated methods. (Primitive types are all specified in camelCase, while object types are all in PascalCase.) We can use all types in print statements:
System.out.println("Hello, World!");
String simian = "monkey"; //initialization for simian
System.out.println("simian: " + simian);
int maxCompiles = 200;
System.out.println("I've compiled this code less than " + maxCompiles + " times.");
double confidence = 92.3;
System.out.println("I'm " + confidence + "% confident that I'll enjoy programming in Java.");
$ javac *.java
$ java CodeToRun
Hello, World!
simian: monkey
I've compiled this code less than 200 times.
I'm 92.3% confident that I'll enjoy programming in Java.
In Python, we'd have to use the str() method to concatenate strings with other types. Java automatically converts non-string values to strings when they are needed, for example in concatenation expressions or as the argument to System.out.println. When the value is non-primitive and not a string, Java calls the toString method.
It's important to know that a variable can't be assigned to values with non-matching types. Try adding this line to the end of your main method's body:
maxCompiles = "1000";
Now try to compile your code:
$ javac *.java
CodeToRun.java:12: error: incompatible types
maxCompiles = "1000";
^
required: int
found: String
1 error
It won't even work if you try to redeclare the variable's type. Once a variable has been declared to have a certain type, that type cannot be changed:
String maxCompiles = "1000";
$ javac *.java
CodeToRun.java:12: error: variable maxCompiles
is already defined in method main(String[])
String maxCompiles = "1000";
^
1 error
This happens because the Java compiler reserves different amount of memory for different types, and wants to know ahead of time what that will look like. Most compilers for modern languages make optimization decisions based on this kind of info. None of this is to say that the variable can't be reused! Reassignment to another int works fine:
maxCompiles = 250;
But if a non-integer value is needed, another variable will have to be used.
In your Monkey class's main method, create four variables:
name, a string,isFunny, a boolean,numTeeth, an integer, andtailLength, a double-precision floating-point (double) value.Add a print statement that uses all of the variables in a sentence (or two) about your monkey. Here's what mine does:
$ javac *.java $ java Monkey Susia is a monkey with 17 teeth and a 14.2-inch long tail. It is true that she's funny.
If I change around the values of the variables (but don't change the code in the print statement) then I get something like this:
$ javac *.java $ java Monkey Dominoa is a monkey with 4 teeth and a 26.34-inch long tail. It is false that she's funny.