Add some missing stdlib methods

classlib/src/main/java/org/teavm/classlib/java/lang/TString.java

@@ -15,6 +15,7 @@
  */
 package org.teavm.classlib.java.lang;
 
+import java.util.Iterator;
 import java.util.Locale;
 import org.teavm.classlib.java.io.TSerializable;
 import org.teavm.classlib.java.io.TUnsupportedEncodingException;
@@ -666,4 +667,48 @@ public class TString extends TObject implements TSerializable, TComparable<TStri
     public static String format(Locale l, String format, Object... args) {
         return new TFormatter(l).format(format, args).toString();
     }
+
+    public static String join(CharSequence delimiter, CharSequence... elements) {
+        if (elements.length == 0) {
+            return "";
+        }
+        int resultLength = 0;
+        for (CharSequence element : elements) {
+            resultLength += element.length();
+        }
+        resultLength += elements.length * delimiter.length();
+
+        char[] chars = new char[resultLength];
+        int index = 0;
+        CharSequence firstElement = elements[0];
+        for (int i = 0; i < firstElement.length(); ++i) {
+            chars[index++] = firstElement.charAt(i);
+        }
+        for (int i = 1; i < elements.length; ++i) {
+            for (int j = 0; j < delimiter.length(); ++j) {
+                chars[index++] = delimiter.charAt(j);
+            }
+            CharSequence element = elements[i];
+            for (int j = 0; j < element.length(); ++j) {
+                chars[index++] = element.charAt(j);
+            }
+        }
+
+        return new String(chars);
+    }
+
+    public static String join(CharSequence delimiter, Iterable<? extends CharSequence> elements) {
+        Iterator<? extends CharSequence> iter = elements.iterator();
+        if (!iter.hasNext()) {
+            return "";
+        }
+
+        StringBuilder sb = new StringBuilder();
+        sb.append(iter.next());
+        while (iter.hasNext()) {
+            sb.append(delimiter);
+            sb.append(iter.next());
+        }
+        return sb.toString();
+    }
 }

classlib/src/main/java/org/teavm/classlib/java/nio/charset/TStandardCharsets.java

@@ -0,0 +1,25 @@
+/*
+ *  Copyright 2018 Alexey Andreev.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *       http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package org.teavm.classlib.java.nio.charset;
+
+import org.teavm.classlib.java.nio.charset.impl.TUTF8Charset;
+
+public final class TStandardCharsets {
+    private TStandardCharsets() {
+    }
+
+    public static final TCharset UTF_8 = new TUTF8Charset();
+}

classlib/src/main/java/org/teavm/classlib/java/util/TMap.java

@@ -74,6 +74,15 @@ public interface TMap<K, V> {
         }
     }
 
+    default V putIfAbsent(K key, V value) {
+        V v = get(key);
+        if (v == null) {
+            v = put(key, value);
+        }
+
+        return v;
+    }
+
     default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
         V v = get(key);
         if (v == null) {