Create Java程序设计题.md

docs/java/Java程序设计题.md

+## 泛型的实际应用
+
+### 实现最小值函数
+
+自己设计一个泛型的获取数组最小值的函数.并且这个方法只能接受Number的子类并且实现了Comparable接口。
+
+```java
+//注意：Number并没有实现Comparable
+private static <T extends Number & Comparable<? super T>> T min(T[] values) {
+    if (values == null || values.length == 0) return null;
+    T min = values[0];
+    for (int i = 1; i < values.length; i++) {
+        if (min.compareTo(values[i]) > 0) min = values[i];
+    }
+    return min;
+}
+```
+
+测试：
+
+```java
+int minInteger = min(new Integer[]{1, 2, 3});//result:1
+double minDouble = min(new Double[]{1.2, 2.2, -1d});//result:-1d
+String typeError = min(new String[]{"1","3"});//报错
+```
+
+## 数据结构
+
+### 使用数组实现栈
+
+**自己实现一个栈，要求这个栈具有`push()`、`pop()`（返回栈顶元素并出栈）、`peek()` （返回栈顶元素不出栈）、`isEmpty()`、`size()`这些基本的方法。**
+
+提示：每次入栈之前先判断栈的容量是否够用，如果不够用就用`Arrays.copyOf()`进行扩容；
+
+```java
+public class MyStack {
+    private int[] storage;//存放栈中元素的数组
+    private int capacity;//栈的容量
+    private int count;//栈中元素数量
+    private static final int GROW_FACTOR = 2;
+
+    //TODO：不带初始容量的构造方法。默认容量为8
+    public MyStack() {
+        this.capacity = 8;
+        this.storage=new int[8];
+        this.count = 0;
+    }
+
+    //TODO：带初始容量的构造方法
+    public MyStack(int initialCapacity) {
+        if (initialCapacity < 1)
+            throw new IllegalArgumentException("Capacity too small.");
+
+        this.capacity = initialCapacity;
+        this.storage = new int[initialCapacity];
+        this.count = 0;
+    }
+
+    //TODO：入栈
+    public void push(int value) {
+        if (count == capacity) {
+            ensureCapacity();
+        }
+        storage[count++] = value;
+    }
+
+    //TODO：确保容量大小
+    private void ensureCapacity() {
+        int newCapacity = capacity * GROW_FACTOR;
+        storage = Arrays.copyOf(storage, newCapacity);
+        capacity = newCapacity;
+    }
+
+    //TODO：返回栈顶元素并出栈
+    private int pop() {
+        count--;
+        if (count == -1)
+            throw new IllegalArgumentException("Stack is empty.");
+
+        return storage[count];
+    }
+
+    //TODO：返回栈顶元素不出栈
+    private int peek() {
+        if (count == 0){
+            throw new IllegalArgumentException("Stack is empty.");
+        }else {
+            return storage[count-1];
+        }
+    }
+
+    //TODO：判断栈是否为空
+    private boolean isEmpty() {
+        return count == 0;
+    }
+
+    //TODO：返回栈中元素的个数
+    private int size() {
+        return count;
+    }
+
+}
+
+```
+
+验证
+
+```java
+MyStack myStack = new MyStack(3);
+myStack.push(1);
+myStack.push(2);
+myStack.push(3);
+myStack.push(4);
+myStack.push(5);
+myStack.push(6);
+myStack.push(7);
+myStack.push(8);
+System.out.println(myStack.peek());//8
+System.out.println(myStack.size());//8
+for (int i = 0; i < 8; i++) {
+    System.out.println(myStack.pop());
+}
+System.out.println(myStack.isEmpty());//true
+myStack.pop();//报错：java.lang.IllegalArgumentException: Stack is empty.
+```
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