added uebung04 task

uebung04/chess/Knight.java

@@ -0,0 +1,28 @@
+package chess;
+
+import static java.lang.Math.abs;
+
+public class Knight extends Piece {
+  public Knight(Color color, Board board, int row, int col) {
+    super(color, board, row, col);
+  }
+
+  @Override
+  public char charRep() {
+    return getColor() == Color.white ? 'n' : 'N';
+  }
+
+  @Override
+  public String toString() {
+    return "" + getColor() + " knight at (" + getRow() + ", " + getCol() + ")";
+  }
+
+  @Override
+  public boolean canCapture(Piece other) {
+    if (getBoard() != other.getBoard() || getColor() == other.getColor())
+      return false;
+    final int dr = abs(getRow() - other.getRow());
+    final int dc = abs(getCol() - other.getCol());
+    return dr == 2 && dc == 1 || dr == 1 && dc == 2;
+  }
+}