java 面试题

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一. LinkedList 与 ArrayList 的区别?

    1.从插入速度来说，LinkedList只是改变last和next的引用，而ArrayList需要定期判断空间是否需要扩容，需要扩容的时候，调用Array.copyOf整个复制，所以当数据量大的时候，LinkedList的插入速度明显要比ArrayList快很多。

    ArrayList:

public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
}
private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
}
private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
}

    LinkedList:

public boolean add(E e) {
        linkLast(e);
        return true;
}
void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
}

    2.从删除速度来说，LinkedList也只是改变了prev 和 next 的引用，所以理论上来说，速度要快一些,

    而ArrayList需要将数组移位。浪费了很多的时间

    ArrayList:

//删除
//首先有个rangeCheck判断，判断是否超过坐标
//其次将数组移位
//然后将要删除的位置置空
//返回删除的value
public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    LinkedList:

//删除
    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }
//检索要删除的node
    Node<E> node(int index) {
        // assert isElementIndex(index);

        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }
//更改引用
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }

    3.从检索来讲，ArrayList直接通过索引来检索，效率更高，而LinkedList需要把数组分隔，然后判断是从开始找或者从末尾找，效率要比ArrayList低。

    ArrayList:

/**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    LinkedList:

/**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

二. HashMap 与  Hashtable 的 区别?

    1.从插入速度来说  Hashtable要比HashMap要慢，因为Hashtable是加同步锁的，安全的map，HashMap是非安全的    

      HashMap:

//非同步put
public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);
        int hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

    HashTable:

public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry tab[] = table;
        int hash = hash(key);
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                V old = e.value;
                e.value = value;
                return old;
            }
        }

        modCount++;
        if (count >= threshold) {
            // Rehash the table if the threshold is exceeded
            rehash();

            tab = table;
            hash = hash(key);
            index = (hash & 0x7FFFFFFF) % tab.length;
        }

        // Creates the new entry.
        Entry<K,V> e = tab[index];
        tab[index] = new Entry<>(hash, key, value, e);
        count++;
        return null;
    }

    2.从删除速度来说，Hashtable要比HashMap要慢，因为Hashtable是加同步锁的，安全的map，HashMap是非安全的    

    HashMap:

public V remove(Object key) {
        Entry<K,V> e = removeEntryForKey(key);
        return (e == null ? null : e.value);
}

    HashTable:

public synchronized V remove(Object key) {
        Entry tab[] = table;
        int hash = hash(key);
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry<K,V> e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                modCount++;
                if (prev != null) {
                    prev.next = e.next;
                } else {
                    tab[index] = e.next;
                }
                count--;
                V oldValue = e.value;
                e.value = null;
                return oldValue;
            }
        }
        return null;
}

3.从查询速度来说，Hashtable要比HashMap要慢，因为Hashtable是加同步锁的，安全的map，HashMap是非安全的    

HashMap:

public V get(Object key) {
        if (key == null)
            return getForNullKey();
        Entry<K,V> entry = getEntry(key);

        return null == entry ? null : entry.getValue();
}
final Entry<K,V> getEntry(Object key) {
        if (size == 0) {
            return null;
        }

        int hash = (key == null) ? 0 : hash(key);
        for (Entry<K,V> e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))
                return e;
        }
        return null;
}

HashTable:

public synchronized V get(Object key) {
        Entry tab[] = table;
        int hash = hash(key);
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                return e.value;
            }
        }
        return null;
}

三、某个线程进入了死循环，导致其他线程一直等待，这种问题如何解决？

可以通过linux下top命令查看cpu使用率较高的java进程，进而用top -Hp ➕pid查看该java进程下cpu使用率较高的线程。再用jstack命令查看线程具体调用情况，排查问题。

四、判断链表有没有环

可以参考 https://blog.youkuaiyun.com/u010983881/article/details/78896293

五、java限流策略

https://www.cnblogs.com/java1024/p/7725632.html

面试及学习几要素

一.概念

二.使用

三.原理

四.包含哪些组件

五.瓶颈、可以从哪方面优化

六.为什么用这个

七.用这个解决哪些问题

八.和其他类似工具的区别

九.多做笔记多思考，知其然知其所以然

转载于:https://my.oschina.net/u/1178126/blog/1603007