This guide is meant to review some of the key ideas and point out common misconceptions with objects, arrays, and inheritance.
Let’s start with the simple stuff: arrays. This should be review, but the main idea about arrays is that they are the “equivalent” of lists in Python. There are a few differences though.
Common Mistake: Many students forget that, unlike lists in Python, arrays in Java are of fixed length and the length of an array has to be specified when you instantiate it.
Common Mistake: Many students use the Python syntax for getting the length of an array. If I have an array a in Java, I can get the length using a.length.
Note that
.lengthis not a method of an array, but rather an attribute. For strings, getting the length requires using the.length()method (e.g.s.length()).
2-D arrays are a data structure that you will see a lot in 61B, so you should get familiar with them ASAP. At a high level, a 2-D array is an array whose elements are pointers to other arrays. Here’s a diagram of how I like to visualize 2-D arrays.
We looked at a problem in section called flatten, which takes in an int[][] data and returns a 1D array which contains all the elements in data. Here are some of the key takeaways from the problem:
Key Takeaway: Whenever you want to access all the values in a 2-D array, you need to write a nested for-loop. This just means writing one for-loop inside another for-loop.
// An example of a nested-for loop
for (int i = 0; i < 10; i++;) {
for (int j = 0; j < 10; j++;) {
System.out.print(i + j);
}
}
However, as we saw in the section, writing for loops like these can get cumbersome really quickly when you have to keep track of 2 or more different sets of indices. Therefore, we looked at an alternate syntax for writing for-loops:
// data is a 2-D integer array
for (int[] col: data) {
for (int elem: col) {
System.out.print(elem);
}
}
These kind of for-loops are easier to read and often easier to implement because they resemble English. If you were to say out loud what iterating through a 2-D array is in English, it would sound something like “For every column in the 2-D array, do something with each element of the column.” Notice that this is really close to the code above.
Final note on 2-D arrays: Indexing into a 2-D array is no different than indexing into a normal 1-D array. For example, data[0][0] gets you to the first element in the first column of data.
We can create objects by defining classes and instantiating instances of those classes.
Officially, the Golden Rule of Equals (also referred to as Pass by Reference or Pass by Value) states: “Given variables a and b, the assignment statement a = b copies all the bits from b into a.”
I personally hate thinking about bits, so the way I remember this rule is with environment diagrams. When we execute the assignment a = b, we simply copy whatever is in the “box” for b into the box for a. So if b is an integer or any other primitive type, we simply copy the primitive into the “box” for a. If b is bound to an object, then what’s actually in the “box” for b is a pointer, or reference, to the object. Therefore, we just copy the pointer into the “box” for a.
Common Mistake: Many students forget that the Golden Rule of Equals also applies when passing variables in as arguments to a method. If a method takes in an object as a parameter, what actually gets passed in is a pointer to the object that is being passed in.
Key Takeaway: Drawing environment diagrams really helps to see what is actually going on in your program. Use these diagrams so you don’t miss easy points on an exam because you made a careless mistake in your head.
Inheritance refers to the way classes relate to each other. Some classes are children of their superclass and other classes have children classes, or subclasses. Class A is a subclass of class B if A extends B.
Common Mistake: Many students forget that any call to the constructor method of a class begins with a call to the constructor method of its superclass. This makes sense intuitively. If you are trying to make a Beagle, you first need to make a Dog so that you can inherit its methods and attributes.
Note: For the record, although I used
Beaglein the example, German Shepherds are the best dogs and you can’t convince me otherwise.
Dynamic method selection is the process that is run to execute any method call in Java. This topic is a favorite for Midterm 1 questions, so make sure you understand it.
I could list out the steps of dynamic method selection, but it gets a little confusing to follow because there are so many steps. Thus, I’ll show the process of dynamic method selection with a flowchart.
Assuming we are executing a method call dj.play(moBamba):
This diagram assumes that the instantiation of the objects
djandmoBambaare valid (i.e. that their dynamic type is of the same class or a subclass of their static type).
I hope you found this guide helpful! As always, if you have any questions about something I said here (or anything else) please reach out to me and I would be more than happy to answer!