里氏替换原则 定义：子类可以扩展父类的功能，但不能改变父类原有的功能（子类继承父类时，除了添加新的方法和功能外，尽量不要重写父类的方法）以正方形不是长方形为例：在resize方法中，Rectangle类型的参数不能被Square类型的参数所代替，如果进行了替换，则不能得到预期的打印结果Rectanglepublic class Rectangle { private double length; private double width; public double getLength() { return length; } public void setLength(double length) { this.length = length; } public double getWidth() { return width; } public void setWidth(double width) { this.width = width; } }Squarepublic class Square extends Rectangle { @Override public void setLength(double length) { super.setLength(length); super.setWidth(length); } @Override public void setWidth(double width) { super.setWidth(width); super.setLength(width); } }RectangleDemopublic class RectangleDemo { public static void main(String[] args) { //创建长方形对象 Rectangle rectangle = new Rectangle(); rectangle.setLength(20); rectangle.setWidth(10); //扩宽 resize(rectangle); printLengthAndWidth(rectangle); System.out.println("===================="); //创建正方形 Square square = new Square(); square.setWidth(20); resize(square); //在resize方法中，Rectangle类型的参数不能被Square类型的参数所代替，如果进行了替换，则不能得到预期的打印结果 printLengthAndWidth(square); } public static void resize(Rectangle rectangle) { //如果长 > 宽，进行扩宽操作 while (rectangle.getLength() >= rectangle.getWidth()) { rectangle.setWidth(rectangle.getWidth() + 1); } } public static void printLengthAndWidth(Rectangle rectangle) { System.out.println("长：" + rectangle.getLength()); System.out.println("宽：" + rectangle.getWidth()); } }改进：抽象出四边形接口，长方形、正方形实现四边形接口Quadrilateralpublic interface Quadrilateral { /** * 获取长 * * @return */ double getLength(); /** * 获取宽 * * @return */ double getWidth(); }Rectanglepublic class Rectangle implements Quadrilateral { private double length; private double width; public void setLength(double length) { this.length = length; } public void setWidth(double width) { this.width = width; } @Override public double getLength() { return length; } @Override public double getWidth() { return width; } }Squarepublic class Square implements Quadrilateral { private double side; public double getSide() { return side; } public void setSide(double side) { this.side = side; } @Override public double getLength() { return side; } @Override public double getWidth() { return side; } }RectangleDemopublic class RectangleDemo { public static void main(String[] args) { //创建长方形对象 Rectangle rectangle = new Rectangle(); rectangle.setLength(20); rectangle.setWidth(10); resize(rectangle); printLengthAndWidth(rectangle); System.out.println("===================="); //创建正方形对象（此时正方形和长方形不存在父子关系） Square square = new Square(); square.setSide(10); printLengthAndWidth(square); } public static void resize(Rectangle rectangle) { //如果长 > 宽，进行扩宽操作 while (rectangle.getLength() >= rectangle.getWidth()) { rectangle.setWidth(rectangle.getWidth() + 1); } } public static void printLengthAndWidth(Quadrilateral quadrilateral) { System.out.println("长：" + quadrilateral.getLength()); System.out.println("宽：" + quadrilateral.getWidth()); } }