added uebung04 task

uebung04/chess/Piece.java

@@ -0,0 +1,38 @@
+package chess;
+
+public abstract class Piece {
+  private Color color;
+  private Board board;
+  private int row;
+  private int col;
+
+  protected Piece(Color color, Board board, int row, int col) {
+    if (row < 1 || row > 8 || col < 1 || col > 8)
+      throw new IllegalArgumentException("Invalid pos " + row + "/" + col);
+    this.color = color;
+    this.board = board;
+    this.row = row;
+    this.col = col;
+    board.add(this);
+  }
+
+  public Color getColor() {
+    return color;
+  }
+
+  public Board getBoard() {
+    return board;
+  }
+
+  public int getRow() {
+    return row;
+  }
+
+  public int getCol() {
+    return col;
+  }
+
+  public abstract char charRep();
+
+  public abstract boolean canCapture(Piece other);
+}