Java

1. Java

2. Features • strongly-typed language • object-oriented • encapsulation enforced • compiled before executed • more difficult to learn than Python, but safer for large program development and maintenance

3. Hello World! public class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); } }

4. How to run on fissure • save in file: HelloWorld.java • compile with: javac HelloWorld.java • execute with: java HelloWorld • output (sent to screen): Hello World!

5. Contrast to Python • from within the interpreter: print “Hello World!” • or save in file: HelloWorld.py • and execute with: python HelloWorld.py

6. Java: so complicated? • all code must belong to a class • program execution begins in method main() of the “main” class • classes and methods have visibility modifiers (encapsulation) • method return value must be declared

7. … not really. public class HelloWorld { public static void main (String[ ] args) { System.out.println(“Hello World!”); } }

8. Some minor points • class and method bodies must be enclosed by left { and right } brackets • statements must end with a semi-colon ; • no problem, the javac compiler will complain if you forget, and some IDEs help you get it right • good news: indentation not important other than for style

9. Compiler checks these public class HelloWorld { public static void main (String[ ] args) { System.out.println(“Hello World!”); } }

10. Another example class Counter: # from Python Review #1 def __init__(self): self.value = 0 def step(self): self.value += 1 def current(self): return self.value # dynamic typing

11. Testing Counter # Testing the class definition: c = Counter() print “initial value”, c.current() c.step() print “after one step”, c.current() # or ... print “after one step”, c.value # ... because Python does not enforce encapsulation!

12. Counter in Java public class Counter { private int value; public Counter () { this.value = 0; } public void step() { this.value += 1; } public int current() { return this.value; } } // static typing // encapsulation enforced

13. TestCounter in Java public class TestCounter { public static void main (String[] args) { Counter c = new Counter(); System.out.println(“initial value ” + c.current()); c.step(); System.out.println(“after one step ” + c.current()); // but not ... // System.out.println(“after one step ” + c.value); } } // ... because Java enforces encapsulation!

14. Compile, then execute # note the order of compilation javac Counter.java TestCounter.java # class with main() is executed java TestCounter