Java数据类型—StringBuilder与StringBuffer「建议收藏」

    @Test
    public void testPerformance(){ 
   
        String str0 = "hello,world";

        long start = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) { 
   
            str0 += i;
        }
        System.out.println(System.currentTimeMillis() - start);

        StringBuilder sb = new StringBuilder("hello,world");
        long start1 = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) { 
   
            sb.append(i);
        }
        System.out.println(System.currentTimeMillis() - start1);

        StringBuffer sbf = new StringBuffer("hello,world");
        long start2 = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) { 
   
            sbf.append(i);
        }
        System.out.println(System.currentTimeMillis() - start2);
    }

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4

    @Test
    public void testSafe() throws InterruptedException { 
   
        String str0 = "hello,world";

        StringBuilder sb = new StringBuilder(str0);
        for (int i = 0; i < 100; i++) { 
   
            new Thread(() -> { 
   
                try { 
   
                    TimeUnit.MILLISECONDS.sleep(1);
                } catch (InterruptedException e) { 
   
                    e.printStackTrace();
                }
                sb.append("a");
            }).start();
        }


        StringBuffer sbf = new StringBuffer(str0);
        for (int i = 0; i < 100; i++) { 
   
            new Thread(() -> { 
   
                try { 
   
                    TimeUnit.MILLISECONDS.sleep(1);
                } catch (InterruptedException e) { 
   
                    e.printStackTrace();
                }
                sbf.append("a");
            }).start();
        }
        // 等待工作线程运行结束
        while (Thread.activeCount()>2){ 
   

        }
        System.out.println("StringBuilder:"+sb.toString().length());
        System.out.println("StringBuffer:"+sbf.toString().length());
    }

StringBuilder:109
StringBuffer:111

StringBuilder sb1 = new StringBuilder();
StringBuilder sb2 = new StringBuilder(100);

     */
    public StringBuilder() { 
   
        super(16);
    }

    /** * Constructs a string builder with no characters in it and an * initial capacity specified by the {@code capacity} argument. * * @param capacity the initial capacity. * @throws NegativeArraySizeException if the {@code capacity} * argument is less than {@code 0}. */
    public StringBuilder(int capacity) { 
   
        super(capacity);
    }

    /** * Constructs a string builder initialized to the contents of the * specified string. The initial capacity of the string builder is * {@code 16} plus the length of the string argument. * * @param str the initial contents of the buffer. */
    public StringBuilder(String str) { 
   
        super(str.length() + 16);
        append(str);
    }

    /** * Constructs a string builder that contains the same characters * as the specified {@code CharSequence}. The initial capacity of * the string builder is {@code 16} plus the length of the * {@code CharSequence} argument. * * @param seq the sequence to copy. */
    public StringBuilder(CharSequence seq) { 
   
        this(seq.length() + 16);
        append(seq);
    }

/** * Appends the specified string to this character sequence. * <p> * The characters of the {@code String} argument are appended, in * order, increasing the length of this sequence by the length of the * argument. If {@code str} is {@code null}, then the four * characters {@code "null"} are appended. * * @param str a string. * @return a reference to this object. */
public AbstractStringBuilder append(String str) { 
   
    if (str == null)
        return appendNull();
    int len = str.length();
    ensureCapacityInternal(count + len);
    str.getChars(0, len, value, count);
    count += len;
    return this;
}
/** * For positive values of {@code minimumCapacity}, this method * behaves like {@code ensureCapacity}, however it is never * synchronized. * If {@code minimumCapacity} is non positive due to numeric * overflow, this method throws {@code OutOfMemoryError}. */
private void ensureCapacityInternal(int minimumCapacity) { 
   
    // overflow-conscious code
    if (minimumCapacity - value.length > 0) { 
   
        value = Arrays.copyOf(value,
                newCapacity(minimumCapacity));
    }
}

/** * Returns a capacity at least as large as the given minimum capacity. * Returns the current capacity increased by the same amount + 2 if * that suffices. * Will not return a capacity greater than {@code MAX_ARRAY_SIZE} * unless the given minimum capacity is greater than that. * * @param minCapacity the desired minimum capacity * @throws OutOfMemoryError if minCapacity is less than zero or * greater than Integer.MAX_VALUE */
private int newCapacity(int minCapacity) { 
   
    // overflow-conscious code
    int newCapacity = (value.length << 1) + 2;
    if (newCapacity - minCapacity < 0) { 
   
        newCapacity = minCapacity;
    }
    return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
        ? hugeCapacity(minCapacity)
        : newCapacity;
}