Software Design7.1Java on one slidez All objects allocated on heap, via new, garbage collected¾ Primitive types like int, double, boolean exempt• Everything else subclasses Object¾ All variables (non-primitive) are pointers aka references• Can we compare pointers for equality? Is this a problem?z No free functions, everything in a class, inheritance by default¾ Functions and classes can be final, not inheritable from ¾ Static functions like Math.sqrt are like free functions¾ Local variables must be assigned to, instance variables all initialized by default to 0, nullz Containers contain only non-primitive types¾ Conversion between int and Integer can be ugly¾ Use ArrayList and HashMap instead of Vector, HashtableSoftware Design7.2Java on another slidez Public class Foo must be in a file Foo.java¾ Compiled into Java bytecodes, stored in Foo.class• Bytecodes executed inside a JVM: Java Virtual Machine• JVM is architecture specific, often relies on native/C code¾ Helper/non-public classes can be in same file• Keep related/cohesive concepts together• Don’t go overboardz Execution starts with a static main function¾ Any class can have such a function, class invoked specifically via java Foo (runs Foo.main)z The environment is important and essential¾ You need to understand CLASSPATH to leverage JavaSoftware Design7.3From STL to Javaz In STL an iterator is a concept, there are refinements¾ Input, output, forward, bidirectional, random access• A forward iterator is an input iterator and an output iterator• The iterator may be immutable (or const)---read only¾ Refinements not implemented by inheritance, but by design, contract, and subsequently implementation• What happens if you try to implement an STL iterator?z In Java Iterator is an interface (like a base class), similar to Tapestry iterators¾ Collection(s) are required to have iterators, these are used in some operations like max, min, construct vector, …¾ Related to STL as algorithm glue, but very different Software Design7.4WordCount.java, print strings, line #’spublic void print() { Iterator allKeys = myMap.keySet().iterator(); // wordswhile (allKeys.hasNext()) { String key = (String) allKeys.next(); System.out.print(key + "\t");Iterator lines = ((Set) myMap.get(key)).iterator(); while (lines.hasNext()) { System.out.print(lines.next() + " "); } System.out.println(); } }z Differences between Java and Tapestry in practice?¾ Must store current element since next() does two things¾ Must cast since Collections store ObjectsSoftware Design7.5Java inheritancez By default every class can be a base/parent class, every method is polymorphic. To inherit use extends keyword¾ Can change with final keyword (similar to const, but not)¾ A class can extend only one base class (but see interfaces)¾ Public, protected, private similar to C++, what’s not?z A class can be an abstract class, public abstract class Foo¾ Can’t instantiate (no new Foo()), but can extend¾ A method can be abstract, like pure virtual in C++z A class implements any number of interfaces¾ Like ABC, but function prototypes only, no state¾ Subclass must implement all methods of interfaceSoftware Design7.6Modules and Packagesz Java code/modules organized into packages¾ C++ has namespaces, required and now used¾ Java uses packages: corresponds to directory hierarchy¾ We’re using the default package (no name) later we’ll use packages¾ java.util, java.lang, java.io, … are all packagesz The import statement at the beginning of a program doesn’t work like #include, it tells the Java compiler where to look to resolve names¾ Differences from
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