私はJavaプログラミングに非常に慣れていないので、次のようにjarをクラスパスに追加しようとしています:
javac -classpath ~/Downloads/algs4.jar. ThreeSum.java 
しかし、それでも次のような例外が発生します:
ThreeSum.java:38: error: cannot find symbol
                        StdOut.println(a[i] + " " + a[j] + " " + a[k]);
                        ^
  symbol:   variable StdOut
  location: class ThreeSum
ThreeSum.java:62: error: cannot find symbol
        int[] a = In.readInts(args[0]);
                  ^
  symbol:   variable In
  location: class ThreeSum
ThreeSum.java:64: error: cannot find symbol
        Stopwatch timer = new Stopwatch();
        ^
  symbol:   class Stopwatch
  location: class ThreeSum
ThreeSum.java:64: error: cannot find symbol
        Stopwatch timer = new Stopwatch();
                              ^
  symbol:   class Stopwatch
  location: class ThreeSum
ThreeSum.java:66: error: cannot find symbol
        StdOut.println("elapsed time = " + timer.elapsedTime());
        ^
  symbol:   variable StdOut
  location: class ThreeSum
ThreeSum.java:67: error: cannot find symbol
        StdOut.println(cnt);
        ^
  symbol:   variable StdOut
  location: class ThreeSum
6 errors