StringBuilder vs. StringBuffer

AbstractStringBuilder.java

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;
}

StringBuffer.java

public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence {
    
    /* ... */
    
    @Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }
}

StringBuilder.java

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence {
    
    /* ... */
    
    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }
}