constructors
the truth!

Review terms

Local variable: can be used/modified only inside the method it's declared in; example: s1, s2, s2a are local vars in testAll (lines 48–51)); t, a, etc. are params for new Song2 (line 9)).
parameter: A type of local-variable; It gets initialized when the function is called (using the value passed to the function); So don't you initialize it yourself! (Also a best-practice: don't re-assign to a parameter ¹, even though you can.); example: s, space are params for willFit (line 23)); t, a, etc. are params for new Song2 (line 9)).
argument: The value passed to a function; example: 2, 20 are arguments to Math.pow (line 24)); "Abba" is one of the arguments to new Song2 (line 50); 150 is one of the arguments to new Song2 (line 56) and in turn the song that new Song2 returns is one of the arguments to assertEquals (line 54)
field: Declared inside class Foo { … }, but not inside any of its methods/functions.; example: artist, title, isCopyRighted inside class Song2 (lines 2–5 below); While it looks like a local-variable declaration in java, it's very very different! If you make 20 Song2s, there are 20 different titles running around, even when you're not running any of the functions/methods inside Song2.

Constructors

They're just regular functions, but Java sure tries to obscure that with special rules:

name: it must be the same name as the class. (Fair 'nuff.)
return type: don't write it. (Fair 'nuff; it'd be reduncant to say Song Song( String t, …).)
To call: new␣ClassName — think of new␣ as part of the function's name.
implicit variable this made for you: // Song2 this = /* a Song2 object w/ all fields "zero-ish" */; (Don't write it yourself; Java adds it behind your back when you hit "compile". Besides, it kinda begs the question of how you'd write the right-hand-side of the statement.)
implicit return this as the last line. (Don't write it yourself; Java adds it behind your back when you hit "compile".)
Purpose of a constructor: to initialize all fields

A very typical constructor is very boring, doing nothing more than assigning to each field:

    Song2( String t, String a, double l, boolean isC ) {
        // Song2 this = /* a Song2 object w/ all fields "zero-ish" */;  // implicit -- don't write it!
        this.title = t;
        this.artist = a;
        this.length = l;
        this.isCopyrighted = isC;
        // return this;    // implicit -- don't write it!
     }

Functions returning Objects

We will copy/rename class Song into class Song2 (before, after). and then add a method “remix” (starting with test-cases, to figure out exactly what parameters that function needs, and what it should do in corner cases).

 1  class Song2 extends Object120 {
 2      String title;
 3      String artist;
 4      double length;  // running time, in s.
 5      boolean isCopyrighted;
 6      
 7      
 8      // the "constructor":
 9      Song2( String t, String a, double l, boolean isC ) {
10          // Song2 this = /* a Song2 object w/ all fields "zero-ish" */;  // implicit -- don't write it!
11          this.title = t;
12          this.artist = a;
13          this.length = l;
14          this.isCopyrighted = isC;
15          // return this;    // implicit -- don't write it!
16      }
17      
18       
19      /** return whether `s` can be stored using `space` disk-space.
20       * @param s the song to consider
21       * @param space the amount of available space it has to fit into, in B.
22       */
23      static boolean willFit( Song2 s, int space ) {
24          return (s.length/60)*3.0*Math.pow(2,20) <= space;
25      }
26      
27      /** return whether one song is shorter than another.
28       * @return whether s1 is shorter than s2.
29       */
30      static boolean isShorter( Song2 s1, Song2 s2 ) {
31          return (s1.length < s2.length);
32      }
33      
34      /** return a remix of a song.
35       * @return `orig` remixed by D.J. `dj`, who added `newMaterialLength` seconds of ill beatz.
36       */
37      static Song2 remix( Song2 orig, String dj, double newMaterialLength ) {
38          return new Song2( orig.title + " (" + dj + " remix)",
39                            orig.artist + " vs " + dj,
40                            orig.length + newMaterialLength,
41                            false );
42      }
43      
44      
45      
46      static void testAll() {
47          // some examples of Song2 objects:
48          Song2 s1 = new Song2("Old Town Road", "Lil Nas X", 120, true);
49          Song2 s2 = new Song2("Ring Ring", "Abba", 180, true);
50          Song2 s2a = new Song2("Ring Ring", "Abba", 180, true);
51          
52          
53          printTestMsg("remix");
54          assertEquals( new Song2( "Old Town Road (ibarland remix)",
55                                   "Lil Nas X vs ibarland",
56                                   150,
57                                   false ),
58                        remix(s1, "ibarland", 30 ) );
59          assertEquals( new Song2("Ring Ring (saltman remix)", "Abba vs saltman", 180, false),
60                        remix(s2, "saltman", 0 ) );
61          printTestSummary();
62      }
63      
64      public static void main( String[] args ) { testAll(); }
65      
66  }

¹ Pro tip: Java lets you declare your parameters final! E.g. int someFunc( final int n, final String s ) { … } ↩

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constructors the truth!

Review terms

Constructors

Functions returning Objects

constructors
the truth!