Note: Based on jdk11

AbstractStringBuilder

Variable Abstract string creation class

The following interfaces are implemented:

1. Appendable the implementation class of this interface can splice character sequences or single characters. Thread safety is not guaranteed, whether it is implemented by the implementation class. Format using java.util.Formatter, must implement class for appendable
2. CharSequence

Several important member variables:

/**
 * The byte array of string storage data. The capacity can be specified during initialization. The default value is 16
 */
byte[] value;

/**
 * Code 0 is Latin1 1 is Utf-16
 */
byte coder;

/**
 * Actual length of current string
 */
int count;

private static final byte[] EMPTYVALUE = new byte[0];

/**
    *  byte [] Maximum length limit, exceeding VM limit length will cause OOM
    */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

Several important methods:

1. Returns the actual length of the current string, count
public int length() {
        return count;
}
2. Return string capacity
public int capacity() {
        return value.length >> coder;
}
3. Expansion, byte array copy
public void ensureCapacity(int minimumCapacity) {
       if (minimumCapacity > 0) {
                ensureCapacityInternal(minimumCapacity);
      }
}

private void ensureCapacityInternal(int minimumCapacity) {
       // overflow-conscious code
       int oldCapacity = value.length >> coder;
       if (minimumCapacity - oldCapacity > 0) {
               value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity) << coder);
       }
}
private int newCapacity(int minCapacity) {
        // overflow-conscious code
       int oldCapacity = value.length >> coder;
        int newCapacity = (oldCapacity << 1) + 2;
        if (newCapacity - minCapacity < 0) {
                newCapacity = minCapacity;
        }
    int SAFE_BOUND = MAX_ARRAY_SIZE >> coder;
    return (newCapacity <= 0 || SAFE_BOUND - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
}

private int hugeCapacity(int minCapacity) {
        int SAFE_BOUND = MAX_ARRAY_SIZE >> coder;
       int UNSAFE_BOUND = Integer.MAX_VALUE >> coder;
       if (UNSAFE_BOUND - minCapacity < 0) { // overflow
                throw new OutOfMemoryError();
        }
        return (minCapacity > SAFE_BOUND)
            ? minCapacity : SAFE_BOUND;
}

StringBuffer

Thread safe AbstractStringBuilder implementation class

Several important methods:

1. toString() uses the cache mechanism. When toString() is called for the first time, the copy of the real-time generated string will be saved in toStringCache. The next call will return directly. If there is any change, toStringCache will be set to null

StringBuilder

Non thread safe AbstractStringBuilder implementation class

Several important methods:

1. toString() creates a string copy in real time and returns